Negative pole piece, preparation method of negative pole piece and battery

Abstract

The embodiment of the invention provides a negative pole piece, a preparation method of the negative pole piece and a battery, and relates to the technical field of batteries. The negative pole piece comprises a current collector, a first coating and a second coating, the first coating is arranged on at least one surface of the current collector, the second coating is arranged on the surface, away from the current collector, of the first coating, and the first coating comprises carbon nano onions. The first coating with the carbon nano onions is arranged between the current collector and the second coating, the first coating serves as a transition layer, and the second coating serves as a negative electrode active layer. The first coating has elastic deformation and can absorb expansion stress generated by the second coating when the battery is charged and discharged, so that the powder falling phenomenon of the second coating is prevented, the peeling strength between the second coating and the current collector is effectively improved, and the performance of the battery is further improved.

Description

Technical Field

[0001] This application belongs to the field of battery technology, and specifically relates to a negative electrode sheet, a method for preparing the negative electrode sheet, and a battery. Background Technology

[0002] The energy storage industry is entering the era of large-scale energy storage, with increasingly larger battery cell sizes and higher requirements for volumetric energy density. To improve the volumetric energy density of battery cells, current cell designs primarily focus on high compaction and high density of electrodes. By increasing the compaction density and degree of compaction of the electrodes, more electrical energy can be stored within a limited space, thereby increasing the overall energy density of the battery cell. Based on this, thick electrode design has become an important choice. Compared to thin electrodes, thick electrodes can accommodate more active material within the same area, thus increasing the cell's capacity.

[0003] However, the thick electrode design can easily cause powder shedding in the coating of the negative electrode graphite material system. Furthermore, during the charging and discharging process of the battery cell, the volume of the negative electrode graphite layer changes due to the expansion force and breathing effect, which can also easily lead to the risk of powder shedding in the negative electrode graphite layer. Summary of the Invention

[0004] This application provides a negative electrode sheet, a method for preparing the negative electrode sheet, and a battery. By disposing a first coating between the current collector and the second coating, the first coating can suppress the expansion of the second coating, effectively improve the peel strength of the second coating on the current collector, and improve the periodic powder shedding phenomenon of the negative electrode sheet, thereby improving the performance of the battery.

[0005] In a first aspect, this application provides a negative electrode sheet, including a current collector, a first coating and a second coating, wherein the first coating is disposed on at least one surface of the current collector and the second coating is disposed on the surface of the first coating opposite to the current collector; the first coating includes carbon nanotubes.

[0006] This application incorporates a first coating layer between the current collector and the second coating. This first coating serves as a transition layer, enabling it to form a strong interfacial bond with both the current collector and the second coating. Simultaneously, the carbon nanotube onion material itself is capable of elastic deformation, providing a buffer for the second coating. When the second coating undergoes volume changes, the first coating absorbs the stress generated by the volume expansion, effectively solving the problem of easy peeling of the second coating caused by direct application to the current collector and improving the powder shedding phenomenon of the negative electrode sheet.

[0007] In some embodiments, the thickness ratio of the first coating to the second coating is 0.1%-0.2%.

[0008] In some embodiments, the thickness of the first coating is 90nm-100nm; the thickness of the second coating is 50μm-70μm.

[0009] In some embodiments, the first coating comprises a dispersant and a first binder, wherein the mass ratio of carbon nanoparticles, dispersant and first binder is (80-100):(3-8):(1.5-4).

[0010] In some embodiments, the dispersant includes one or more of polyvinylpyrrolidone, polyvinyl alcohol, sodium dodecyl sulfate, sodium dodecylbenzene sulfonate, polyethylene glycol monooctylphenyl ether, and polysorbate, and the first binder includes one or more of sodium carboxymethyl cellulose, polyacrylic acid, sodium polyacrylate, and sodium alginate.

[0011] In some embodiments, the second coating comprises graphite, a conductive agent, and a second binder, wherein the mass ratio of graphite, conductive agent, and second binder is (90-95):(2-3):(6-8).

[0012] In some embodiments, the conductive agent comprises carbon black and carbon nanotubes, wherein the mass ratio of carbon black to carbon nanotubes is 2:1 or 1:1.

[0013] In some embodiments, the mass ratio of styrene-butadiene rubber to sodium carboxymethyl cellulose ranges from 3:2 to 2:1.

[0014] In some implementations, the contact angle of the current collector is 22°-28°.

[0015] In some implementations, the surface roughness of the current collector is ≤3.5μm.

[0016] In some embodiments, the negative electrode further includes a third coating located between the current collector and the first coating, the third coating serving to connect the current collector and the first coating.

[0017] Secondly, this application provides a method for preparing a negative electrode sheet according to the above embodiments, comprising the steps of: coating a first slurry onto at least one surface of a current collector, and obtaining a first coating after a first drying, wherein the first slurry comprises carbon nanoparticles; coating a second slurry onto the surface of the first coating away from the current collector, and obtaining a second coating after a second drying; and rolling the current collector, the first coating and the second coating to obtain a negative electrode sheet.

[0018] This application forms a first coating by coating a first slurry containing carbon nanoparticles onto a current collector, and then coating a second slurry, which is a negative electrode active material, onto the side of the first coating away from the current collector to form a second coating. The first coating can utilize the multilayer structure of the carbon nanoparticles to absorb the expansion stress generated by the second coating during charging and discharging, thereby improving the peel strength of the second coating on the current collector, reducing the powder shedding phenomenon of the negative electrode sheet, and thus improving the performance of the battery.

[0019] In some embodiments, the preparation of the first slurry includes: dissolving carbon nanoparticles and a dispersant in a first solvent, adding a first binder after a first mixing treatment, and obtaining the first slurry after a second mixing treatment and a first filtration; wherein the mass ratio of carbon nanoparticles, dispersant and first binder is (80-100):(3-8):(1.5-4), the first solvent includes water, and the solid content of the first slurry is 8.5wt%-9.5wt%.

[0020] In some embodiments, the viscosity of the first slurry is 3500 Pa·s-4500 Pa·s.

[0021] In some embodiments, the average particle size of the first slurry is 120 nm to 180 nm.

[0022] In some implementations, the coefficient of variation of the first slurry is ≤10%.

[0023] In some implementations, the settling rate of the first slurry over 24 hours is ≤5%.

[0024] In some embodiments, the first mixing process includes mixing with a mixer for 1-5 hours at a temperature of 50°C-60°C.

[0025] In some embodiments, the first mixing process includes dispersion treatment using a high-shear emulsifier for a treatment time of 0.5 h to 2 h and a treatment temperature of 30 °C to 60 °C.

[0026] In some embodiments, the first mixing process includes ultrasonic dispersion for a time of 60 seconds to 30 minutes and a temperature of 20°C to 50°C.

[0027] In some embodiments, the second mixing process includes mixing with a mixer for 15-25 minutes at a temperature of 50°C-60°C.

[0028] In some embodiments, the preparation of the second slurry includes: dissolving graphite, a conductive agent, and a second binder in a second solvent, and obtaining the second slurry after a third mixing treatment and a second filtration; wherein the mass ratio of graphite, conductive agent, and second binder is (90-95):(2-3):(6-8), the second solvent includes water, and the solid content of the second slurry is 51wt%-53wt%.

[0029] In some embodiments, the viscosity of the second slurry is 8000 Pa·s-10000 Pa·s.

[0030] In some implementations, the settling rate of the second slurry is ≤3% after 1 hour.

[0031] In some embodiments, the first drying includes pre-drying, with a drying temperature of 50°C-70°C, an air velocity of 2m / s-4m / s, and a drying time of 2min-10min, wherein the solvent residue rate after pre-drying is 50%-60%.

[0032] In some embodiments, high-temperature curing is used for drying. The gas atmosphere during drying is a nitrogen atmosphere with an oxygen content of ≤5%, the drying temperature is 100℃-120℃, and the drying time is 5min-30min. The solvent residue rate after high-temperature curing is ≤0.5%.

[0033] In some embodiments, before coating the first slurry onto at least one surface of the current collector and obtaining the first coating after a first drying process, the preparation method further includes: pretreating the current collector; wherein the pretreating step includes: immersing the current collector in an alkaline solution for alkaline washing, the alkaline solution including at least one of sodium hydroxide and sodium dodecyl sulfate, the alkaline washing time being 1 min to 5 min; immersing the alkaline-washed current collector in an acidic solution for acid washing, the acidic solution including sulfuric acid, the acid washing time being 30 s to 3 min; and drying the alkaline-washed and acid-washed current collector for 1 min to 5 min at a drying temperature of 70°C to 90°C.

[0034] In some embodiments, before drying the current collector after alkaline washing and acid washing, the method further includes hydrolyzing the silane coupling agent under acidic conditions to obtain a hydrolysate solution, coating the hydrolysate solution on at least one surface of the current collector after alkaline washing and acid washing, and then performing a third drying treatment; wherein the concentration of the silane coupling agent is 0.2wt%-0.8wt%.

[0035] Thirdly, this application also provides a battery, including a positive electrode, a separator, an electrolyte, and a negative electrode as described in the above embodiments.

[0036] This application involves placing a first coating with carbon nanotubes between a current collector and a second coating. The first coating serves as a transition layer, and the second coating is the negative electrode active layer. The first coating exhibits elastic deformation, which can absorb the expansion stress generated by the second coating during battery charging and discharging. This effectively improves the peel strength between the second coating and the current collector, thereby preventing powder shedding from the negative electrode and thus improving battery performance. Attached Figure Description

[0037] Figure 1 This is a schematic diagram of a negative electrode sheet provided in an embodiment of this application; Figure 2 This is a flowchart of a method for preparing a negative electrode sheet provided in an embodiment of this application; Figure 3This is a flowchart of a method for preparing the second slurry provided in an embodiment of this application; Figure 4 This is a flowchart of a first hybrid processing method provided in an embodiment of this application; Figure 5 This is a flowchart of a second hybrid processing method provided in an embodiment of this application; Figure 6 This is a flowchart of a method for preparing the second slurry provided in an embodiment of this application; Figure 7 This is a flowchart of a first drying method provided in an embodiment of this application; Figure 8 This is a flowchart of a method for preparing a negative electrode sheet provided in an embodiment of this application; Figure 9 This is a flowchart of a pretreatment process for a current collector provided in an embodiment of this application; Figure 10 This is a flowchart of a pretreatment process for a current collector provided in an embodiment of this application; Figure 11 This is a flowchart of a second drying method provided in an embodiment of this application.

[0038] Figure Labels 10-Negative electrode sheet; 11-Current collector; 12-First coating; 13-Second coating. Detailed Implementation

[0039] The embodiments of this application are described below with reference to the accompanying drawings.

[0040] It should be understood that the described embodiments are merely some, not all, of the embodiments in this application. All other embodiments obtained by those skilled in the art based on the embodiments in this application without inventive effort are within the scope of protection of this application.

[0041] The terminology used in the embodiments of this application is for the purpose of describing particular embodiments only and is not intended to be limiting of this application. The singular forms “a,” “the,” and “the” used in the embodiments of this application and the appended claims are also intended to include the plural forms unless the context clearly indicates otherwise.

[0042] It should be understood that the term "and / or" used in this document is merely a description of the same field in the related objects, indicating that three relationships can exist. For example, A and / or B can represent: A alone, A and B simultaneously, and B alone. Additionally, the character " / " in this document generally indicates that the preceding and following related objects have an "or" relationship.

[0043] It should be understood that the terms "first," "second," etc., used in this application are for distinguishing purposes only and should not be construed as indicating or implying relative importance or order.

[0044] As the energy storage industry moves towards the era of large-scale energy storage, the requirements for volumetric energy density are increasing with the size of battery cells. To improve the volumetric energy density of cells, current cell designs primarily focus on high compaction and high density of electrodes. By increasing the compaction density and degree of compaction of the electrodes, more electrical energy can be stored within a limited space, thereby improving the overall energy density of the cell. Based on this, thick electrode design has become an important choice. Compared to thin electrodes, thick electrodes can carry more active material in the same area, thus increasing the capacity of the cell.

[0045] However, the thick electrode design makes it easier for the negative electrode graphite material system to shed powder, and during the charging and discharging process of the battery cell, the negative electrode graphite layer is also at risk of shedding powder due to the expansion force and breathing effect of the negative electrode graphite layer.

[0046] To address the aforementioned issues, this application provides a negative electrode sheet, a method for preparing the negative electrode sheet, and a battery. By coating a carbon-based transition layer onto the surface of the negative electrode current collector, and then coating a graphite negative electrode active layer onto the carbon-based transition layer, the peel strength of the electrode powder on the current collector can be improved, the powder shedding phenomenon of the thick graphite negative electrode can be reduced, thereby improving the cycle stability of the electrode and effectively enhancing the overall performance of the battery.

[0047] Figure 1 This is a schematic diagram of the negative electrode 10 provided in an embodiment of this application, as shown below. Figure 1 As shown, this application provides a negative electrode 10, including a current collector 11, a first coating 12 and a second coating 13. The first coating 12 is disposed on at least one surface of the current collector 11, and the second coating 13 is disposed on the surface of the first coating 12 facing away from the current collector 11. The first coating 12 includes carbon nanotubes (CNO).

[0048] The negative electrode 10 includes a current collector 11, a first coating 12, and a second coating 13. The current collector 11 serves as the electrode substrate, providing mechanical support for the first coating 12 and the second coating 13, ensuring the integrity and stability of the electrode structure. The current collector 11 acts as an electron transport channel; during battery charging and discharging, electrons migrate rapidly between the external circuit and the active material through the current collector 11, thereby achieving battery energy conversion.

[0049] A first coating 12 is disposed on at least one surface of the current collector 11, and a second coating 13 is disposed on the surface of the first coating 12 facing away from the current collector 11, i.e., the first coating 12 is located between the current collector 11 and the second coating 13. The second coating 13 is disposed on the surface of the first coating 12 facing away from the current collector 11 and completely covers the surface of the first coating 12 facing away from the current collector 11.

[0050] The first coating 12 can be located on one surface of the current collector 11 or on two surfaces opposite to the current collector 11. When the first coating 12 is located on two surfaces opposite to the current collector 11, the second coating 13 is coated on both sides of the first coating 12 of the current collector 11. This embodiment of the application is illustrated by taking the example of the first coating 12 being located on one surface of the current collector 11.

[0051] In this embodiment, the second coating 13 is a negative electrode active layer used to support ion transport and participate in the construction of electron conduction pathways. The second coating 13 undergoes significant volume changes during charging and discharging, leading to the pulverization of the active material on the second coating 13 and even its detachment from the current collector 11.

[0052] Based on this, this application provides a first coating 12 made of carbon nanotube onion material. The first coating 12 serves as a transition layer, capable of forming a strong interfacial bond with the current collector 11 and the second coating 13. Simultaneously, the carbon nanotube onion material itself can undergo elastic deformation, providing a buffer for the second coating 13. When the second coating 13 undergoes volume changes, the first coating 12 can absorb the stress generated by the volume expansion of the second coating 13, thereby effectively solving the problem of easy peeling of the second coating 13 when directly coated on the current collector 11, and improving the powder shedding phenomenon of the second coating 13.

[0053] Specifically, the carbon nanotube onion has a multi-layer nested structure, which allows the surface of the first coating 12 to have more active sites and a larger specific surface area, thereby enabling more atoms or molecules in the first coating 12 to come into contact with the current collector 11 and the second coating 13, thus increasing the interfacial bonding ability.

[0054] Meanwhile, there are certain interaction forces between the layers of the carbon nanotube onion, which can cause the layers to slide or deform relative to each other. When the first coating 12 is subjected to external force, the layers can undergo relative displacement and deformation, thereby enabling the first coating 12 to undergo elastic deformation, which can effectively absorb the expansion force of the second coating 13.

[0055] In one possible implementation, the thickness ratio of the first coating 12 and the second coating 13 is 0.1%-0.2%.

[0056] In this embodiment, by setting the thickness ratio between the first coating 12 and the second coating 13, the first coating 12 is made thick enough in the negative electrode 10 to provide a buffer for the second coating, while avoiding the first coating 12 being too thick and hindering the transport of electrons and ions between the second coating 13 and the current collector 11, and maintaining the content of active material in the second coating 13, thereby ensuring the charge and discharge performance of the battery. It is understood that... Figure 1 This is merely a schematic diagram provided for this application and does not represent the actual thickness ratio between the current collector 11, the first coating 12, and the second coating 13.

[0057] In one embodiment, the thickness ratio of the first coating 12 and the second coating 13 can be 0.1%, 0.15%, 0.2%, etc., or any value within the above range, without specific limitation. If the thickness ratio of the first coating 12 and the second coating 13 is too small, it indicates that the thickness of the first coating 12 is relatively thin, resulting in insignificant elastic deformation of the first coating 12, which may affect the absorption of expansion stress of the second coating 13; if the thickness ratio of the first coating 12 and the second coating 13 is too large, it indicates that the first coating 12 is too thick, which may affect the electron migration efficiency between the second coating 13 and the current collector 11.

[0058] In one embodiment, the thickness of the first coating 12 is between 90nm and 100nm. For example, the thickness of the first coating can be 90nm, 92nm, 94nm, 96nm, 98nm, 100nm, etc., or any value within the above range, which is not limited here.

[0059] In one embodiment, the thickness of the second coating 13 is between 50 μm and 70 μm. For example, the thickness of the second coating can be 50 μm, 55 μm, 60 μm, 65 μm, 70 μm, etc., or any value within the above range, which is not limited here.

[0060] In one embodiment, the current collector 11 can be a copper foil, with a thickness ranging from 6 μm to 12 μm. This is to avoid deformation or breakage due to external forces caused by an excessively thin current collector 11 (less than 6 μm), and to avoid excessive resistance to electron transport due to an excessively thick current collector 11 (greater than 12 μm). This improves the electron transport efficiency of the current collector while ensuring its mechanical strength. It is understood that the current collector 11 can also be other metal materials such as aluminum foil, or even metal composite materials; no limitation is made here.

[0061] In one possible implementation, the first coating 12 includes carbon nanotubes, a dispersant, and a first binder.

[0062] In this embodiment, the dispersant can be adsorbed on the surface of carbon nanoparticles, changing the charge distribution on the particle surface, thereby preventing particles from getting close to each other and causing agglomeration, so that the carbon nanoparticles can be uniformly dispersed in the system, thereby improving the uniformity of the thickness of the first coating 12 coated on the current collector 11.

[0063] The first binder fills the gaps between the carbon nanoparticles (like onion particles) to form a bonding network that connects the carbon nanoparticles together, ensuring the stability of the first coating 12 structure and maintaining its elasticity. Simultaneously, the addition of the first binder enables the first coating 12 to adhere to the surface of the current collector 11, enhancing the bonding force between them. For example, during battery charging and discharging, even if the second coating 13 experiences significant volume changes and stress, the first coating 12 can maintain a stable bond with the current collector 11 through the action of the first binder, preventing it from peeling off.

[0064] In one possible implementation, the mass ratio of carbon nanoparticles, dispersant, and first binder is (80-100):(3-8):(1.5-4), which ensures that the carbon nanoparticles are uniformly dispersed while also ensuring that a high-performance bonding network is formed inside the first coating 12. This makes the first coating 12 structurally stable and elastic, thereby fully absorbing the expansion stress generated by the second coating 13 during battery charging and discharging, ensuring the stability of the battery structure and the cycle stability of the battery.

[0065] In this embodiment, the first coating 12 is prepared by reasonably proportioning carbon nanotube onion, dispersant and first binder, so that the first coating 12 can fully perform its function, effectively solve the problem of easy peeling and powdering of the second coating 13, and ensure the stability of its own structure, thereby improving the cycle life and reliability of the battery.

[0066] In one possible implementation, the dispersant includes one or more of polyvinylpyrrolidone (PVP K30), sodium dodecyl sulfate (SDS), sodium dodecylbenzene sulfonate (LAS), polyvinyl alcohol (PVA), polyethylene glycol monooctylphenyl ether (Triton X-100), and polysorbate (Tween); the first binder includes one or more of sodium carboxymethyl cellulose (CMC), polyacrylic acid (PAA), sodium polyacrylate (PAAS), and sodium alginate, thereby ensuring the stability of the first coating 12 structure while achieving efficient dispersion of carbon nanotube onions.

[0067] In one embodiment, the dispersant may be polyvinylpyrrolidone, thereby ensuring the dispersibility of the dispersant in water.

[0068] In one embodiment, the first coating 12 includes carbon nano-onion, polyvinylpyrrolidone, and sodium carboxymethyl cellulose. The mass ratio of carbon nano-onion, polyvinylpyrrolidone, and sodium carboxymethyl cellulose can be (80:3:1.5), (85:4:2), (90:5:3), (100:8:4), etc., or any value within the above range. No specific limitation is made here.

[0069] Preferably, the mass ratio of carbon nanotube onion, polyvinylpyrrolidone, and sodium carboxymethyl cellulose is 100:4:2.5.

[0070] In one possible implementation, the second coating 13 comprises graphite, a conductive agent, and a second binder.

[0071] In this embodiment, graphite is used as the active material in the second coating 13 to store and release ions during battery charging and discharging, thereby realizing the conversion of electrode energy.

[0072] The conductive agent fills the gaps between graphite particles, thereby increasing the contact points between graphite particles, reducing the internal resistance of the battery, enabling rapid electron transfer during charging and discharging, and improving the battery's charging and discharging performance.

[0073] The second binder is used to bond the current collector 11 and graphite particles together, improving the stability of the second coating 13. The functional groups of the second binder can form hydrogen bonds or chemical bonds with the functional groups in the first coating 12, thereby enhancing the interfacial bonding between the first coating 12 and the second coating 13, preventing some of the powder from the second coating 13 from falling off the first coating 12, and thus extending the battery's lifespan. In one possible embodiment, the mass ratio of graphite, conductive agent, and second binder is (90-95):(2-3):(6-8), which can both ensure the conductivity of the second coating 13 and improve the stability of the second coating 13 adhering to the first coating 12, thereby improving the overall performance of the battery.

[0074] In one embodiment, the second coating 13 can use graphite as the negative electrode active material, or it can use lithium titanate (LTO) as the negative electrode active material, as long as the energy conversion of the battery can be guaranteed. It is understood that the type of negative electrode active material of the second coating 13 can be various, and this application does not limit it.

[0075] In one possible implementation, the conductive agent includes one or more of carbon black (Super P), carbon nanotubes (CNT), graphene, acetylene black, and vapor-grown carbon fibers; the second binder includes one or more of styrene-butadiene rubber (SBR) and sodium carboxymethyl cellulose.

[0076] In one embodiment, the second coating 13 comprises graphite, carbon black, carbon nanotubes, styrene-butadiene rubber, and sodium carboxymethyl cellulose. Carbon black and carbon nanotubes serve as conductive agents, with a mass ratio of 2:1 or 1:1. Styrene-butadiene rubber and sodium carboxymethyl cellulose serve as a second binder, with a mass ratio of (3:2) to (2:1).

[0077] For example, the mass ratio of graphite, carbon black, carbon nanotubes, styrene-butadiene rubber and sodium carboxymethyl cellulose can be (90:2:1:3:2), (92:2:1:3:2), (94:2:1:3:2), (95:2:1:3:2), (90:1:1:2:1), (92:1:1:2:1), (94:1:1:2:1), (95:1:1:2:1), etc., or any value within the above range, without specific limitations.

[0078] In this embodiment, carbon black can fill the gaps between graphite particles and disperse around them. Carbon nanotubes complement carbon black, both contributing to improving the conductivity of the second coating 13. Furthermore, the long fiber structure of the carbon nanotubes can wrap around the surface of the graphite particles, thereby improving the overall structural stability of the second coating 13 and reducing the occurrence of problems such as active material shedding due to volume changes in the second coating 13 during battery charging and discharging.

[0079] Sodium carboxymethyl cellulose and styrene-butadiene rubber work together to bond the mixture of graphite and conductive agent to the current collector. They can also absorb the stress caused by volume changes when the battery is touched during charging and discharging, thereby effectively reducing the shedding of active material from the current collector and extending the battery's lifespan.

[0080] In one embodiment, the carboxyl groups on the surface of the first coating 12 can form hydrogen bonds with sodium carboxymethyl cellulose, thereby enhancing the interfacial bonding between the first coating 12 and the second coating 13. In another embodiment, the carboxyl groups on the surface of the first coating 12 can form hydrogen bonds with styrene-butadiene rubber, thereby enhancing the interfacial bonding between the first coating 12 and the second coating 13.

[0081] In one possible implementation, the contact angle of the current collector 11 is 22°-28°.

[0082] In this embodiment, by reducing the contact angle of the current collector 11 from 80°-90° to 22°-28°, the hydrophilicity of the current collector 11 is improved, increasing the contact area between the carbon nanoparticles (like onion particles) and the copper foil in the first slurry used to form the first coating 12, and enhancing the interfacial bonding between the first coating 12 and the current collector 11. Simultaneously, the lower contact angle ensures that the active functional groups (e.g., -OH, -COOH) introduced during the pretreatment of the current collector 11 are fully exposed, allowing the oxygen-containing groups (e.g., -COOH, -OH) on the surface of the first coating 12 to undergo hydrogen bonding, forming chemical anchoring points, enhancing the chemical bonding between the first coating 12 and the current collector 11, and further improving the stability of the negative electrode 10 structure.

[0083] In one embodiment, the current collector 11 may be pretreated by at least one step of alkaline washing and acid washing activation to reduce the contact angle of the surface of the current collector 11, thereby improving the bonding strength between the current collector 11 and the first coating 12.

[0084] In one possible implementation, the surface roughness of the current collector 11 is ≤3.5μm.

[0085] In this embodiment, by controlling the surface roughness of the current collector 11, the bonding strength of the first coating on the current collector 11 is ensured, preventing excessive surface roughness of the current collector 11 from causing pits in the appearance of the negative electrode 10 after coating, thus affecting the yield of the negative electrode 10. Simultaneously, the contact area between the surface of the current collector 11 and the first coating 12 is ensured, allowing the carbon nanoparticles to embed into the rough surface of the current collector 11, preventing the current collector 11 from being too smooth and causing the first coating 12 to detach, thereby ensuring the bonding strength between the two.

[0086] In one possible implementation, the negative electrode 10 further includes a third coating located between the current collector 11 and the first coating 12. The third coating includes a silane coupling agent and is used to connect the current collector 11 and the first coating 12, thereby improving the bonding strength between the current collector 11 and the first coating 12 and preventing the first coating 12 from detaching from the current collector 11, which would cause a decrease in the yield of the negative electrode 10.

[0087] In one embodiment, the silane coupling agent is γ-aminopropyltriethoxysilane (KH550), which includes amino and triethoxysilyl groups. The amino group can physically bond with the carboxyl groups on the surface of the first coating 12 through hydrogen bonding. Simultaneously, the triethoxysilyl group can be hydrolyzed to produce silanol, which further reacts with the hydroxyl groups on the surface of the current collector 11 to form covalent bonds, thereby achieving the connection of the third coating to both the current collector 11 and the first coating 12.

[0088] Figure 2This is a flowchart of a method for preparing the negative electrode 10 provided in this application embodiment, combined with... Figure 1 and Figure 2 As shown, this application also provides a method for preparing a negative electrode 10, used to prepare the negative electrode 10 described in the above embodiments, comprising the following steps: Step S1: The first slurry is coated on at least one surface of the current collector 11, and after a first drying, a first coating 12 is obtained, wherein the first slurry comprises carbon nanotubes. Step S2: The second slurry is coated on the surface of the first coating 12 that is away from the current collector 11, and after a second drying process, the second coating 13 is obtained; Step S3: The current collector 11, the first coating 12 and the second coating 13 are rolled to obtain the negative electrode sheet 10.

[0089] In this embodiment, the current collector 11 can be made of copper foil. A prepared first slurry is coated onto the copper foil, covering the outer surface of the current collector 11. After a first drying process, a first coating 12 of a certain thickness is formed on the surface of the current collector 11. Then, a prepared second slurry is coated onto the first coating 12 away from the outer surface of the current collector 11, covering the outer surface of the first coating 12 and the current collector 11. After a second drying process, a second coating 13 of a certain thickness is formed on the first coating 12. The current collector 11 with the first coating 12 and the second coating 13 is then rolled to obtain the negative electrode sheet 10. In this embodiment, the first slurry includes carbon nanotubes. The layered structure of the carbon nanotubes has elastic deformation, providing a buffer for the second coating 13, so that the first coating 12 prepared from the first slurry can absorb the expansion stress generated by the second coating 13 during battery charging and discharging, thereby improving the peel strength of the second coating 13 on the current collector 11 and thus improving the powder shedding phenomenon of the negative electrode sheet 10.

[0090] In one embodiment, the thickness of the first coating 12 is adjusted according to specific circumstances so that the first coating 12 can completely absorb the expansion stress generated by the second coating 13, thereby improving the powder shedding phenomenon of the negative electrode sheet 10. The thickness of the second coating 13 can also be set according to the actual capacity of the battery, thereby ensuring the battery capacity.

[0091] In one embodiment, in step S1, the first slurry can be coated onto the current collector 11 using a slit coater. Specifically, the coating speed of the slit coater should be set to 4 m / min-10 m / min, the die gap to 0.04 mm-0.08 mm, and the back roller pressure to 0.2 MPa-0.3 MPa, thereby ensuring that the wet film thickness of the first coating 12 is between 200 nm and 300 nm, the dry film thickness is between 50 nm and 100 nm, and ensuring the uniformity of the first slurry coating on the surface of the current collector 11.

[0092] In one embodiment, in step S2, the second slurry can be coated onto the surface of the first coating 12 using a wide-slit coating machine. Specifically, the coating speed of the wide-slit coating machine should be set to 3m / min-9m / min, and the die gap should be 0.1mm-0.3mm to ensure that the wet film thickness of the second coating 13 after coating is between 160μm-180μm and the dry film thickness is between 55μm-60μm. This ensures that the second slurry can completely cover the first coating 12 and avoids the current collector 11 being exposed, which would reduce the area of ​​the effective active material and affect the battery capacity.

[0093] Figure 3 This is a flowchart of a method for preparing the second slurry provided in an embodiment of this application, combined with... Figure 1 and Figure 3 As shown, in one possible implementation, prior to step S1, the preparation of the first slurry includes: Step S02: Dissolve carbon nanoparticles and dispersant in a first solvent, add a first binder after a first mixing treatment, and obtain a first slurry after a second mixing treatment and a first filtration; wherein, the mass ratio of carbon nanoparticles, dispersant and first binder is (80-100):(3-8):(1.5-4), the first solvent includes water, and the solid content of the first slurry is 8.5wt%-9.5wt%.

[0094] In this embodiment, when preparing the first slurry, carbon nanoparticles and a dispersant are first dissolved in a first solvent and mixed, and then a first binder is added for a second mixing process. This allows the dispersant to preferentially adsorb onto the surface of the carbon nanoparticles, thus solving the problem of carbon nanoparticle agglomeration. The first binder is added after the mixture of carbon nanoparticles and dispersant is dispersed to build a stable network structure, preventing the carbon nanoparticles from being encapsulated by the first binder and causing agglomeration of the first slurry. Furthermore, this solution prepares the first slurry by rationally proportioning carbon nanoparticles, a dispersant, and a first binder. This allows the dispersant to reduce the particle size of the carbon nanoparticles, ensuring sufficient dispersion, while the first binder effectively adjusts the viscosity of the carbon nanoparticle mixture, allowing the carbon nanoparticles to be fully coated onto the current collector 11 during subsequent coating. This ensures both the uniformity of the first coating 12 and the carbon nanoparticle content in the first coating 12, effectively absorbing the expansion stress of the second coating 13.

[0095] In one embodiment, the dispersant in the first slurry includes polyvinylpyrrolidone (PVP), the first binder includes sodium carboxymethyl cellulose (CMC), and the first solvent includes deionized water. First, oxidized carbon nanotubes (carboxyl density ≥ 0.8 mmol / g, particle size 5 nm-50 nm), PVP (molecular weight 40,000), sodium carboxymethyl cellulose (degree of substitution 0.7), and deionized water are prepared. The mass ratio of carbon nanotubes, PVP, and sodium carboxymethyl cellulose is 100:4:2.5. Then, carbon nanotubes and PVP are added to the deionized water. A first mixing treatment is then performed to obtain a uniformly mixed first mixture. The first binder is then added for a second mixing treatment to adjust the viscosity of the first mixture, forming a second mixture. The second mixture is then filtered first to obtain the first slurry.

[0096] In one possible implementation, the viscosity of the first slurry is 3500 Pa·s-4500 Pa·s.

[0097] In this embodiment of the application, by adding a first binder to the first mixture and then performing a second mixing treatment, the viscosity of the first slurry can be adjusted to 3500 Pa·s-4500 Pa·s, thereby meeting the flowability requirements of the first slurry and enabling the first slurry to adapt to the coating process. This avoids the first slurry having poor flowability due to excessively high viscosity, which would affect the uniformity of the thickness during coating. At the same time, it ensures the stability of the first coating 12 on the current collector 11 and prevents the first coating 12 from falling off the current collector 11 during battery charging and discharging due to excessively low viscosity.

[0098] In one possible implementation, the average particle size (D50) of the first slurry is 120 nm to 180 nm.

[0099] In one possible implementation, the coefficient of variation (CV) value of the first slurry is ≤10%.

[0100] In this embodiment, the addition of a dispersant allows it to adsorb onto the surface of the carbon nanoparticles (onion particles) to form a steric hindrance layer, effectively preventing agglomeration between the carbon nanoparticles. After a first mixing treatment and a second mixing treatment, the particle distribution in the second mixture is made uniform. Following a first filtration, agglomerates or impurities larger than or equal to 1 μm are removed from the mixture, thereby maintaining the average particle size (D50) of the first slurry between 120 nm and 180 nm and ensuring that the coefficient of variation (CV) value of the first slurry is ≤10%.

[0101] This application adds a dispersant to carbon nanotube onions and performs a first mixing treatment, then adds a first binder and performs a second mixing treatment, and finally filters the mixture to control the average particle size (D50) and coefficient of variation (CV) of the first slurry, thereby enabling sufficient contact between the particles of the first slurry, reducing the internal resistance of the first coating 12, and improving the charge and discharge efficiency; at the same time, it can avoid the agglomeration of the first slurry particles, which would lead to uneven coating.

[0102] In one embodiment, the average particle size (D50) of the first slurry can be detected by a laser particle size analyzer.

[0103] In one possible implementation, the settling rate of the first slurry after standing for 24 hours is ≤5%, which means that the first slurry does not need to be frequently stirred or redispersed, and can also ensure the uniformity of the first slurry coating on the current collector 11, thereby reducing production interruption time and reducing manufacturing costs.

[0104] Figure 4 This is a flowchart of a first hybrid processing method provided in an embodiment of this application, combined with... Figure 1 and Figure 4 As shown, in one possible implementation, in step S02, the first mixing process includes: Step S021: Use a mixer to perform the mixing process for 1-5 hours at a temperature of 50-60℃.

[0105] In one possible implementation, in step S02, the first mixing process further includes: Step S022: Dispersion treatment is carried out using a high-shear emulsifier for 0.5h-2h and at a temperature of 30℃-60℃.

[0106] In one possible implementation, in step S02, the first mixing process further includes: Step S023: Ultrasonic dispersion treatment is adopted, the treatment time is 60s-30min, and the treatment temperature is 20℃-50℃.

[0107] Figure 5 This is a flowchart of a second hybrid processing method provided in an embodiment of this application, combined with... Figure 1 and Figure 5 As shown, in one possible implementation, in step S02, the second mixing process includes: Step S024: Use a mixer to perform the mixing process for 15-25 minutes at a temperature of 50-60℃.

[0108] In this embodiment, the first mixing process can disperse the first mixture through at least one of steps S021, S022, and S023, thereby improving the uniformity of the first slurry. Then, after adding the first binder, step S024 is performed, thereby ensuring that the first slurry can stably adhere to the first coating 12 while avoiding agglomeration of the first slurry after adding the first binder, which would lead to uneven thickness of the first coating 12.

[0109] In one embodiment, the first mixing process includes: firstly, stirring the mixture of carbon nano-onion and polyvinylpyrrolidone (PVP) using a planetary mixer at a speed of 1000 rpm for 30 minutes to initially mix the carbon nano-onion, PPV, and deionized water, ensuring that PPV is uniformly coated on the surface of the carbon nano-onion particles. Then, transferring the stirred mixture of carbon nano-onion and PPV to a high-shear emulsifier for dispersion, wherein the dispersion speed is 12000 rpm, the stator gap is 0.1 mm, and the dispersion time is 60 minutes. The high shear force can break up the particle agglomerates of the mixture, thereby refining the particle size. Next, ultrasonic dispersion is performed at a power of 200W, with a pulse pattern of 5 seconds on and 2 seconds off, for a processing time of 30 minutes, further breaking up the small agglomerates. This pulse mode also prevents the particles in the first mixture from re-agglomerating, thus achieving dispersion of the first mixture.

[0110] In one embodiment, the second mixing process after adding sodium carboxymethyl cellulose to the first mixture includes: stirring the first mixture with added sodium carboxymethyl cellulose using a mixer at a speed of 500 rpm for 20 min to adjust the viscosity of the mixture to 3500 Pa·s-4500 Pa·s, thereby obtaining the second mixture. This ensures the fluidity of the second mixture while also maintaining the content of carbon nanoparticles in the first coating 12.

[0111] In one embodiment, the first filtration is performed by filtering the second mixture after mixing and stirring through a 1 μm filter membrane to remove agglomerates or impurities of 1 μm or larger from the mixture, thereby keeping the average particle size (D50) of the first slurry between 120 nm and 180 nm and keeping the coefficient of variation (CV) value of the first slurry ≤10%.

[0112] Figure 6 This is a flowchart of a method for preparing the second slurry provided in an embodiment of this application, combined with... Figure 1 and Figure 6 As shown, in one possible implementation, prior to step S2, the preparation of the second slurry includes: Step S03: Dissolve graphite, conductive agent and second binder in a second solvent, and obtain a second slurry after a third mixing treatment and a second filtration; wherein the mass ratio of graphite, conductive agent and second binder is (90-95):(2-3):(6-8), the second solvent includes water, and the solid content of the second slurry is 51wt%-53wt%.

[0113] In this embodiment, in step S03, a second coating 13 is prepared by rationally proportioning graphite, a conductive agent, and a second binder. This ensures both the conductivity of the second coating 13 and its stable adhesion to the first coating 12, thereby improving the overall performance of the battery. This solution ensures the uniform dispersion of the prepared second slurry and maintains its conductivity by subjecting the mixture of graphite, conductive agent, and second binder to a third mixing process and a second filtration. It is understood that the second solvent is deionized water.

[0114] In one embodiment, the second adhesive can adjust the viscosity of the prepared second slurry according to actual needs, so that the second slurry has a certain fluidity and can ensure that the second coating 13 is stably adhered to the first coating 12. In this way, while improving the uniformity of the second slurry coating on the first coating 12, it can also ensure the stability of the second coating 13 on the first coating 12 after coating.

[0115] In one embodiment, the conductive agent includes carbon black and carbon nanotubes, and the second binder includes styrene-butadiene rubber and sodium carboxymethyl cellulose. Specifically, artificial graphite with an average particle size (D50) of 15 μm and a compacted density of 1.8 g / cm³ is first prepared. 3 The mixture comprises carbon black and carbon nanotubes, styrene-butadiene rubber and sodium carboxymethyl cellulose, and water. The mass ratio of artificial graphite, conductive agent, and second binder is (90-95):(2-3):(6-8). In the conductive agent, the mass ratio of carbon black to carbon nanotubes is 2:1. In the second binder, the mass ratio of styrene-butadiene rubber to sodium carboxymethyl cellulose is (3:2) to (2:1).

[0116] Next, artificial graphite, carbon black, and carbon nanotubes are added to 55%-80% deionized water and stirred using a high-shear emulsifier at 8000 rpm for 30 minutes to allow initial mixing of the graphite, carbon black, and carbon nanotubes in the deionized water. The carbon black fills the gaps between graphite particles, reducing contact resistance, while the carbon nanotubes complement the carbon black, thus improving the overall conductivity of the second slurry. Then, the remaining 20%-45% deionized water is added, and the mixture is stirred for 15 minutes to obtain a third mixture, further improving the uniformity of the second slurry. Sodium carboxymethyl cellulose is then added to the third mixture, and the mixture is stirred using a mixer at 1000 rpm for 15 minutes to obtain a fourth mixture. In the above embodiment, stirring allows sodium carboxymethyl cellulose to fully contact the graphite surface particles, creating steric hindrance and preventing graphite particle aggregation. Sodium carboxymethyl cellulose also acts as a suspension stabilizer to ensure that the sedimentation rate of the second slurry is ≤3% after standing for 1 hour, thus preventing stratification of the second slurry before coating and resulting in uneven coating. Styrene-butadiene rubber is then added to the fourth mixture, and the mixer is adjusted to 800 rpm for 10 minutes to adjust the viscosity of the fourth mixture, forming the fifth mixture. The fifth mixture is then filtered a second time to obtain the second slurry.

[0117] In one possible implementation, the viscosity of the second slurry is 8000 Pa·s-10000 Pa·s.

[0118] In this embodiment, by adding a second binder to the third mixture and stirring it multiple times, the viscosity of the second slurry can be adjusted to 8000 Pa·s-10000 Pa·s, thereby meeting the fluidity requirements of the second slurry and enabling it to adapt to the coating process. This avoids poor fluidity due to excessively high viscosity of the second slurry, which would affect the uniformity of the coating thickness. At the same time, it ensures that the second coating 13 can stably adhere to the first coating 12, preventing the second coating 13 from falling off during charging and discharging due to its own volume change because the viscosity of the second slurry is too low.

[0119] In one possible implementation, the settling rate of the second slurry is ≤3% after 1 hour, which means that the second slurry does not need to be frequently stirred or redispersed, and the uniformity of the second slurry coating on the first coating layer 12 can be guaranteed, thereby reducing production interruption time and manufacturing costs.

[0120] In one embodiment, the second filtration is performed by filtering the fifth mixture after mixing and stirring through a 150-mesh sieve.

[0121] Figure 7 This is a flowchart of a first drying method provided in an embodiment of this application, combined with... Figure 1 and Figure 7 As shown, in one possible implementation, in step S1, the first drying includes: Step S11: Pre-drying is performed at a temperature of 50℃-70℃, a wind speed of 2m / s-4m / s, and a drying time of 2min-10min. The solvent residue rate after pre-drying is 50%-60%.

[0122] In this embodiment, pre-drying allows the first slurry to evaporate slowly at low temperature, enabling the solvent to diffuse evenly from the coating surface to the interior. This avoids direct high-temperature drying, which would cause the internal solvent to vaporize too quickly and lead to stress concentration, resulting in coating cracking or peeling, thus improving the stability of the first coating 12 on the current collector 11.

[0123] In one possible implementation, step S1, the first drying process further includes: Step S12: High-temperature curing is used for drying. The gas atmosphere during drying is a nitrogen atmosphere with an oxygen content of ≤5%. The drying temperature is 100℃-120℃ and the drying time is 5min-30min. The solvent residue rate after high-temperature curing is ≤0.5%.

[0124] In this embodiment, after step S11, 40%-50% of the solvent remains. This remaining solvent can be completely removed by high-temperature curing, resulting in a solid content of nearly 100% for the first coating 12, forming a dense structure. It is understood that high-temperature curing in a nitrogen atmosphere (oxygen content ≤5%) effectively prevents the oxidation of carbon nanotube onions at high temperatures and avoids the decomposition of sodium carboxymethyl cellulose, thereby ensuring the chemical stability of the first coating 12.

[0125] In the embodiments of this application, the first drying can be achieved by drying the first slurry through at least one of steps S11 and S12, or step S11 can be performed first and then step S12.

[0126] In one embodiment, after step S12, the first coating 12, after the first drying, is naturally cooled to room temperature to ensure the stability of the first coating 12 structure and prevent the first coating 12 from cracking due to excessive temperature difference. The room temperature mentioned in this embodiment is within the temperature range of 20℃-30℃, and the room temperature mentioned in this application is the same as this.

[0127] Figure 8 This is a flowchart of a method for preparing a negative electrode sheet provided in an embodiment of this application. Figure 9 This is a flowchart of a pretreatment process for a current collector provided in an embodiment of this application, combined with... Figure 1 , Figure 8 and Figure 9As shown, in one possible implementation, before step S1, the preparation method further includes: Step S01: Pre-treat the current collector 11.

[0128] In this embodiment of the application, by pre-treating the current collector 11, the bonding force between the first coating 12 and the current collector 11 is significantly improved, thus preventing the first coating 12 from detaching from the current collector 11 and causing the negative electrode sheet 10 to fail.

[0129] Step S01 includes: Step S011: Immerse the current collector 11 in an alkaline solution for alkaline washing. The alkaline solution includes at least one of sodium hydroxide (NaOH) and sodium dodecyl sulfate. The alkaline washing time is 1 min to 5 min.

[0130] Step S012: Immerse the alkaline-washed current collector 11 in an acidic solution for acid washing. The acidic solution includes sulfuric acid (H2). S O4), the pickling time is 30s-3min; Step S013: Dry the current collector 11 after alkali washing and acid washing for 1 min to 5 min, and the drying temperature is 70℃ to 90℃.

[0131] In this embodiment, the current collector 11 is alkaline-washed to remove oil stains from its surface, thereby effectively reducing the contact angle of the current collector 11 and enhancing the wettability of the first slurry coating on it. The alkaline-washed current collector 11 is then activated by acid washing to remove the oxide layer on its surface and simultaneously form Cu-OH bonds on its surface, allowing for chemical bonding with the functional groups in the first coating 12, thus enhancing the adhesion between the current collector 11 and the first coating 12. Furthermore, the combination of alkaline washing and acid washing of the current collector 11 neutralizes any residual alkaline solution after the initial washing, preventing alkaline contamination of subsequent coatings. Finally, the current collector 11 is dried to prevent moisture from interfering with the bonding between it and the first coating 12.

[0132] In one embodiment, in step S01, a 1020-type electrolytic copper foil with a thickness of 6μm-12μm is first placed in an alkaline solution formed by 2wt%-8wt% sodium hydroxide and 0.2wt%-1.0wt% sodium dodecyl sulfate. Then, it is treated with ultrasound (300W, 28kHz) for 1min-5min. Next, the treated current collector 11 is placed in a 0.5wt%-3wt% sulfuric acid (H2SO4) solution and treated with ultrasound (300W, 28kHz) for 30s-3min. It is understood that the linear velocity of the ultrasonic probe is 2m / min-10m / min during both the alkaline and acidic solutions. Then, the current collector 11 is dried at a temperature of 70℃-90℃ for 1min-5min, with an air velocity of 2m / s, to ensure that the surface moisture content of the current collector 11 is ≤0.1%.

[0133] Figure 10 This is a flowchart of a pretreatment process for a current collector provided in an embodiment of this application, combined with... Figure 1 and Figure 10 As shown, in one possible implementation, before step S013, the preprocessing step further includes: Step S014: The silane coupling agent is hydrolyzed under acidic conditions to obtain a hydrolyzed solution. The hydrolyzed solution is coated on at least one surface of the current collector 11 after alkali washing and acid washing, and then a third drying treatment is performed. The concentration of the silane coupling agent is 0.2wt%-0.8wt%.

[0134] In this embodiment of the application, by applying a hydrolysis solution containing a silane coupling agent to at least one surface of the current collector 11 after alkaline washing and acid washing, the functional groups on the silane coupling agent can bond with both the functional groups on the first coating and the functional groups on the current collector 11, thereby enhancing the bonding ability between the current collector 11 and the first coating 12. In one embodiment, the silane coupling agent is first dissolved in an aqueous solution of ethanol (C2H6O) with a pH of 4.5, and then dipped in the solution at 60°C for 5 seconds. Step S013 is then performed to allow the third coating to adhere to the current collector 11. It is understood that the volume ratio of ethanol to water in the aqueous ethanol solution is 3:1. In this embodiment, the silane coupling agent is γ-aminopropyltriethoxysilane. The amino group of γ-aminopropyltriethoxysilane can bind with the carboxyl groups on the surface of the first coating 12 through hydrogen bonding. Simultaneously, the triethoxysilyl group of the silane coupling agent can be hydrolyzed to produce silanol, which further reacts with the hydroxyl groups on the surface of the current collector 11 to form covalent bonds, enhancing the bonding ability between the current collector 11 and the first coating 12. Figure 11 This is a flowchart of a second drying method provided in an embodiment of this application, combined with... Figure 1 and Figure 11As shown, in one possible implementation, in step S2, the second drying includes: Step S21: The first stage of drying is used for drying treatment. The drying temperature is 50℃-70℃, the air velocity is 1.5m / s-3m / s, and the drying time is 2min-5min. The solvent residue rate after the first stage of drying is 80%-90%.

[0135] In this embodiment, the first stage of drying allows the surface of the second slurry to evaporate slowly at low temperature, so that the solvent diffuses evenly from the coating surface to the interior. This avoids the stress concentration caused by excessively rapid vaporization of the internal solvent due to direct high-temperature drying, which could lead to coating cracking or peeling, and improves the stability of the second coating 13 on the first coating 12.

[0136] In one possible implementation, in step S2, the second drying process further includes: Step S22: The second stage of drying is used for drying treatment. The drying temperature is 80℃-100℃, the air velocity is 3m / s-6m / s, and the drying time is 5min-15min.

[0137] In this embodiment of the application, after step S21, there is still 80%-90% solvent remaining. A second stage of drying can be performed by increasing the temperature and wind speed to accelerate the evaporation of the solvent inside the second slurry. At the same time, the microporous structure of the skin layer of the second slurry is used to achieve uniform overflow of the solvent, avoiding internal stress accumulation that could lead to coating cracking.

[0138] In one possible implementation, in step S2, the second drying process further includes: Step S23: The third stage of drying is used for drying treatment. The drying temperature is 100℃-115℃, the air velocity is 2m / s-4m / s, and the drying time is 3min-10min. The solvent residue rate after the third stage of drying is ≤0.5%.

[0139] In this embodiment of the application, after performing step S22, there is still some solvent residue. Therefore, it is necessary to increase the temperature and wind speed to further remove the remaining solvent in the second slurry, thereby enhancing the bonding force between the second coating 13 and the first coating 12.

[0140] This solution performs gradient-segmented drying of the second slurry coated on the first coating 12 through steps S21, S22, and S23. By precisely controlling the temperature, wind speed, and time at each stage, the solvent evaporation rate and coating stress release are dynamically matched, which significantly reduces the cracking rate and powdering rate of the second coating 13 and improves the adhesion between the second coating 13 and the first coating 12, thereby improving the peel strength of the second coating 13 on the current collector 11.

[0141] In one embodiment, after the second slurry is applied to the first coating layer 12, a first drying stage is performed to pre-dry the surface of the second slurry, wherein the drying temperature is 60°C, the drying air velocity is 2 m / s, and the drying time is 3 min. Then, a second drying stage is performed to slowly dry the interior of the second slurry, wherein the drying temperature is 90°C, the drying air velocity is 4 m / s, and the drying time is 8 min. Finally, a third drying stage is performed to deeply dry the second slurry, ensuring that the solvent residue rate of the second slurry is ≤0.5%, wherein the drying temperature is 110°C, the drying air velocity is 3 m / s, and the drying time is 5 min.

[0142] In one embodiment, in step S3, the current collector 11, the first coating 12, and the second coating 13 can be compacted using a twin-roll calender, wherein the rolling speed is 3 m / s-7 m / s. When the twin-roll calender rolls the current collector 11, the first coating 12, and the second coating 13, the first coating 12 can undergo elastic compression, so that the second coating 13 can be tightly bonded to the current collector 11.

[0143] In one embodiment, when the current collector 11, the first coating 12 and the second coating 13 are rolled using a twin-roll calender, stepwise pressurization can be used to avoid the second coating 13 from peeling off due to high pressure at one time.

[0144] In one embodiment, the linear pressure of the first pressurization during the step-by-step pressurization can be from 10 MPa to 15 MPa, and the linear pressure of the first pressurization can be from 15 MPa to 25 MPa. The specific values ​​are not limited in this application.

[0145] In one embodiment, when the current collector 11, the first coating 12, and the second coating 13 are rolled using a twin-roll calender, the compaction density is 1.6 g / cm³-1.7 g / cm³. This ensures the mechanical strength and conductivity of the electrode while reserving buffer space for the volume expansion of the second coating 13, thus avoiding structural damage or performance degradation due to excessive compaction.

[0146] In one embodiment, after step S3, the obtained negative electrode sheet 10 needs to be slit by a laser slitting machine with a wavelength of 1064nm (power of 50W). The slitting width matches the battery model. For example, the slitting width can be 100nm or 150nm. The speed of the machine during slitting is 100mm / s to ensure that there are no burrs on the edge of the slit negative electrode sheet 10 and to ensure that the amount of powder falling off the negative electrode sheet 10 during slitting is ≤0.1mg / cm.

[0147] This application also provides a battery, including a positive electrode, a separator, an electrolyte, and the negative electrode 10 described in the above embodiments.

[0148] This application involves placing a first coating 12 with carbon nanotubes between a current collector 11 and a second coating 13. The first coating 12 serves as a transition layer, and the second coating 13 is the negative electrode active layer. The first coating 12 has elastic deformation, which can absorb the expansion stress generated by the second coating 13 during battery charging and discharging, thereby preventing the second coating 13 from shedding powder and effectively improving the peel strength between the second coating 13 and the current collector 11, thus improving the battery performance.

[0149] The present application will be further illustrated below with specific embodiments and comparative examples.

[0150] Example 1: This embodiment provides a method for preparing a negative electrode sheet, the method comprising the following steps: (1) Copper foil pretreatment: First, a 6μm thick copper foil was immersed in a solution of 5wt% sodium hydroxide and 0.5wt% sodium dodecyl sulfate, and then ultrasonically treated for 3 minutes at a temperature of 48℃-52℃. Next, the copper foil was immersed in a 1wt% sulfuric acid solution and ultrasonically treated for 1 minute at a temperature of 25℃. Then, the copper foil was coated with an ethanol-water solution containing 0.5wt% γ-aminopropyltriethoxysilane, where the ethanol-to-water ratio was 3:1 and the pH was 4.5. Finally, the treated copper foil was dried at 80℃ for 2 minutes.

[0151] (2) Preparation of the first slurry for the first coating: First, carbon nanotube onion, polyvinylpyrrolidone, and sodium carboxymethyl cellulose were weighed in a mass ratio of 100:4:2.5. Then, the carbon nanotube onion and polyvinylpyrrolidone were added to deionized water, and the mixture was stirred for 30 min, subjected to high-shear dispersion for 60 min, and ultrasonically dispersed for 30 min. Next, sodium carboxymethyl cellulose was added to the dispersed mixture, and the mixture was stirred for 20 min. The mixture was then filtered through a 1 μm filter membrane to obtain the first slurry, which had a solids content of 9%.

[0152] (3) First slurry coating and curing: The first slurry prepared in step (2) is coated onto the surface of the copper foil, and the thickness of the first slurry after coating is 250 nm. Then, the copper foil coated with the first slurry is dried for 3 min at a drying temperature of 60 °C and a wind speed of 3 m / s. Then, it is cured at high temperature for 5 min under a nitrogen atmosphere at a curing temperature of 120 °C. Then, it is naturally cooled to room temperature to form a copper foil with a first coating, wherein the thickness of the first coating is 100 nm.

[0153] (4) Preparation of the second slurry for the second coating: First, weigh artificial graphite, carbon black, carbon nanotubes, styrene-butadiene rubber, and sodium carboxymethyl cellulose in a mass ratio of 91:2:1:4:2. Then, add the graphite, carbon black, and carbon nanotubes to 55%-80% deionized water and disperse under high shear for 30 minutes. Next, add the remaining deionized water to the mixture and stir for 15 minutes. Then, add sodium carboxymethyl cellulose and stir for another 15 minutes. Finally, add styrene-butadiene rubber and stir for 10 minutes. The resulting mixture is then passed through a 150-mesh sieve to obtain a second slurry, wherein the solid content of the second slurry is 52%.

[0154] (5) Coating and drying of the second slurry: The second slurry prepared in step (4) is coated onto the surface of the first coating, and the thickness of the second slurry after coating is 170 μm. Then, the second slurry undergoes a first drying treatment at a temperature of 60°C, an air velocity of 2 m / s, and a time of 3 min. Then, a second drying treatment is performed at a temperature of 90°C, an air velocity of 4 m / s, and a time of 8 min. Then, a third drying treatment is performed at a temperature of 110°C, an air velocity of 3 m / s, and a time of 5 min to form a second coating, wherein the thickness of the second coating is 58 μm.

[0155] (6) Roll forming: The copper foil with the first coating and the second coating is rolled to achieve a compaction density of 1.65 g / cm³.

[0156] (7) Slicing: The rolled electrode sheets are then slit to form negative electrode sheets.

[0157] Example 2: This embodiment provides a method for preparing a negative electrode sheet. Except for step (2), in which the mass ratio of carbon nanotube onion, polyvinylpyrrolidone and sodium carboxymethyl cellulose is 70:4:2.5, the other process steps are the same as in Example 1. Example 3: This embodiment provides a method for preparing a negative electrode sheet. Except for step (2), in which the mass ratio of carbon nanotube onion, polyvinylpyrrolidone and sodium carboxymethyl cellulose is 80:4:2.5, the other process steps are the same as in Example 1.

[0158] Example 4: This embodiment provides a method for preparing a negative electrode sheet. Except for step (2), in which the mass ratio of carbon nanotube onion, polyvinylpyrrolidone and sodium carboxymethyl cellulose is 90:4:2.5, the other process steps are the same as in Example 1.

[0159] Example 5: This embodiment provides a method for preparing a negative electrode sheet. Except for step (2), in which the mass ratio of carbon nanotube onion, polyvinylpyrrolidone and sodium carboxymethyl cellulose is 110:4:2.5, the other process steps are the same as in Example 1.

[0160] Comparative Example 1: This comparative example provides a method for preparing a negative electrode sheet. The method involves preparing a bare copper foil by step (1) in Example 1, and then directly coating the second slurry prepared by step (4) in Example 1 onto the bare copper foil. The negative electrode sheet is obtained by the segmented drying described in step (5) and steps (6) and (7).

[0161] Comparative Example 2: This comparative example provides a method for preparing a negative electrode sheet. Except that the carbon nanotubes in step (2) are replaced with carbon nanotubes, the other process steps are the same as in Example 1.

[0162] Comparative Example 3: This comparative example provides a method for preparing a negative electrode sheet. The first slurry prepared in step (2) of Example 1 is mixed with the second slurry prepared in step (4) and coated onto the copper foil prepared in step (1). Then, steps (5), (6) and (7) are performed to form a negative electrode sheet.

[0163] Performance testing: (1) Test of the peel strength of the negative electrode active layer in the negative electrode sheet: The peel strength of the negative electrode sheets of Examples 1-5 and Comparative Examples 1-3 was tested.

[0164] (2) Capacity retention test of negative electrode: The capacity retention rate of batteries prepared from the negative electrode sheets of Examples 1-5 and Comparative Examples 1-3 after being formed and tested at a low rate and cycled 300 times at a charge / discharge rate of 1C / 1C, where 1C refers to the battery charging C-rate (charge / discharge rate) of 1C and the battery discharging C-rate (charge / discharge rate) of 1C.

[0165] Table 1. Performance test results of Examples 1-5 and Comparative Examples 1-3:

[0166] (1) The battery with the negative electrode sheet prepared in Example 4 had a capacity retention rate of 96.80% after 300 cycles, while the batteries with the negative electrode sheet formed by directly coating the second slurry onto bare copper foil in Comparative Example 1, the negative electrode sheet formed by the first slurry prepared with carbon nanotubes in Comparative Example 2, and the negative electrode sheet formed by coating the copper foil with a mixture of the first and second slurries in Example 1, had capacity retention rates of only 89.50%, 92.20%, and 91.50% respectively after 300 cycles. This shows that by introducing carbon nanotubes to prepare the first coating between the copper foil and the second coating, the cycle stability of the negative electrode sheet can be effectively improved.

[0167] (2) Based on the peel strength tests conducted in Example 4 and Comparative Examples 1, 2, and 3, the peel strength of the negative electrode active layer in Example 4 was 65 N / m, while the peel strengths of the negative electrode active layers in Comparative Examples 1, 2, and 3 were only 10 N / m, 42 N / m, and 40 N / m, respectively. This indicates that by introducing carbon nanotubes to prepare the first coating located between the copper foil and the second coating, the peel strength of the negative electrode active layer in the negative electrode sheet can be effectively improved, reducing the risk of powder shedding from the negative electrode sheet during battery charging and discharging, and improving the stability of battery operation.

[0168] (3) As can be seen from Examples 1-5, both excessively high and excessively low mass fraction of carbon nano-onion will result in low peel strength of the negative electrode active layer in the negative electrode sheet and poor capacity retention.

[0169] This application illustrates the detailed process flow of the present invention through the above embodiments, but this application is not limited to the above detailed process flow, that is, it does not mean that the present invention must rely on the above detailed process flow to be implemented. Those skilled in the art should understand that any improvements to this application, equivalent substitutions of the raw materials of the product of this application, addition of auxiliary components, selection of specific methods, etc., all fall within the protection scope and disclosure scope of this application.

Claims

1. A negative electrode sheet, characterized in that, It includes a current collector, a first coating and a second coating, wherein the first coating is disposed on at least one surface of the current collector and the second coating is disposed on the surface of the first coating opposite to the current collector, and the first coating includes carbon nanotubes.

2. The negative electrode sheet according to claim 1, characterized in that, The thickness ratio of the first coating to the second coating is 0.1%-0.2%.

3. The negative electrode sheet according to claim 2, characterized in that, The thickness of the first coating is 90nm-100nm, and the thickness of the second coating is 50μm-70μm.

4. The negative electrode sheet according to claim 1, characterized in that, The first coating comprises a dispersant and a first binder, wherein the mass ratio of the carbon nanoparticles, the dispersant and the first binder is (80-100):(3-8):(1.5-4).

5. The negative electrode sheet according to claim 4, characterized in that, The dispersant includes one or more of polyvinylpyrrolidone, polyvinyl alcohol, sodium dodecyl sulfate, sodium dodecylbenzene sulfonate, polyethylene glycol monooctylphenyl ether, and polysorbate, and the first binder includes one or more of sodium carboxymethyl cellulose, polyacrylic acid, sodium polyacrylate, and sodium alginate.

6. The negative electrode sheet according to claim 1, characterized in that, The second coating comprises graphite, a conductive agent, and a second binder, wherein the mass ratio of the graphite, the conductive agent, and the second binder is (90-95):(2-3):(6-8).

7. The negative electrode sheet according to claim 6, characterized in that, The conductive agent comprises carbon black and carbon nanotubes, wherein the mass ratio of the carbon black to the carbon nanotubes is 2:1 or 1:1; the second binder comprises styrene-butadiene rubber and sodium carboxymethyl cellulose, wherein the mass ratio of the styrene-butadiene rubber to the sodium carboxymethyl cellulose ranges from (3:2) to (2:1).

8. The negative electrode sheet according to claim 1, characterized in that, The contact angle of the current collector is 22°-28°, and / or the surface roughness of the current collector is ≤3.5μm.

9. The negative electrode sheet according to claim 1, characterized in that, The negative electrode sheet further includes a third coating located between the current collector and the first coating, the third coating comprising a silane coupling agent.

10. A method for preparing a negative electrode sheet, the negative electrode sheet according to any one of claims 1-9, characterized in that, Including the following steps: The first slurry is coated on at least one surface of the current collector, and the first coating is obtained after a first drying process, wherein the first slurry comprises carbon nanotube onion; The second slurry is coated onto the surface of the first coating that is away from the current collector, and the second coating is obtained after a second drying process. The negative electrode sheet is obtained by rolling the current collector, the first coating and the second coating.

11. The method for preparing the negative electrode sheet according to claim 10, characterized in that, The preparation of the first slurry includes: The carbon nanoparticles and dispersant are dissolved in a first solvent, and after a first mixing treatment, a first binder is added. After a second mixing treatment and a first filtration, the first slurry is obtained. The mass ratio of the carbon nanoparticles, the dispersant and the first binder is (80-100):(3-8):(1.5-4), the first solvent includes water, and the solid content of the first slurry is 8.5wt%-9.5wt%.

12. The method for preparing the negative electrode sheet according to claim 11, characterized in that, It meets at least one of the following characteristics: The viscosity of the first slurry is 3500 Pa·s-4500 Pa·s; The average particle size of the first slurry is 120nm-180nm; The coefficient of variation of the first slurry is ≤10%; The settling rate of the first slurry after 24 hours is ≤5%.

13. The method for preparing the negative electrode sheet according to claim 11 or 12, characterized in that, The first mixing process includes at least one of the following: The mixing process is carried out using a mixer for 1-5 hours at a temperature of 50-60℃. The dispersion process is carried out using a high-shear emulsifier for 0.5-2 hours at a temperature of 30-60℃. Ultrasonic dispersion was used, with a processing time of 60s-30min and a processing temperature of 20℃-50℃. The second mixing process includes mixing using a mixer for 15-25 minutes at a temperature of 50-60°C.

14. The method for preparing the negative electrode sheet according to claim 10, characterized in that, The preparation of the second slurry includes: The graphite, conductive agent, and second binder are dissolved in a second solvent, and after a third mixing treatment and a second filtration, the second slurry is obtained; wherein the mass ratio of the graphite, the conductive agent, and the second binder is (90-95):(2-3):(6-8), the second solvent includes water, and the solid content of the second slurry is 51wt%-53wt%.

15. The method for preparing the negative electrode sheet according to claim 14, characterized in that, The viscosity of the second slurry is 8000 Pa·s-10000 Pa·s; And / or, the settling rate of the second slurry after 1 hour is ≤3%.

16. The method for preparing the negative electrode sheet according to claim 10, characterized in that, The first drying includes, The drying process is carried out by pre-drying at a temperature of 50℃-70℃, a wind speed of 2m / s-4m / s, and a drying time of 2min-10min. The solvent residue rate after pre-drying is 50%-60%. And / or, high-temperature curing is used for drying, the gas atmosphere during drying is a nitrogen atmosphere with an oxygen content of ≤5%, the drying temperature is 100℃-120℃, and the drying time is 5min-30min, wherein the solvent residue rate after high-temperature curing is ≤0.5%.

17. The method for preparing the negative electrode sheet according to claim 10, characterized in that, Before coating the first slurry onto at least one surface of the current collector and obtaining the first coating after a first drying process, the preparation method further includes: pretreating the current collector, the pretreating step including: The current collector is immersed in an alkaline solution for alkaline washing, the alkaline solution including at least one of sodium hydroxide and sodium dodecyl sulfate, and the alkaline washing time is 1 min to 5 min; The current collector, after being washed with alkali, is immersed in an acidic solution, including sulfuric acid, for acid washing for 30 seconds to 3 minutes. The current collector after alkali washing and acid washing is dried for 1 min to 5 min at a temperature of 70℃ to 90℃.

18. The method for preparing the negative electrode sheet according to claim 17, characterized in that, Before drying the current collector after alkaline washing and acid washing, the pretreatment step includes hydrolyzing the silane coupling agent under acidic conditions to obtain a hydrolysate solution, coating the hydrolysate solution onto at least one surface of the current collector after alkaline washing and acid washing, and then performing a third drying treatment; wherein the concentration of the silane coupling agent is 0.2wt%-0.8wt%.

19. A battery, characterized in that, It includes a positive electrode, a separator, an electrolyte, and a negative electrode as described in any one of claims 1-9.

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