Negative plate capable of avoiding super-thick edge and manufacturing method thereof

Abstract

The invention discloses a negative plate capable of avoiding super-thick edge and a manufacturing method thereof, the manufacturing method comprises the following steps: S1, preparing negative electrode slurry: mixing graphite and a silicon-carbon composite material as an active substance, and preparing the negative electrode slurry with the solid content of 45-55wt%; s2, pulse type ultrasonic dispersion: treating the slurry by adopting intermittent pulse ultrasonic waves, wherein the ratio of working time to intermittent time is (2-3): 1; s3, surface tension gradient control: adding a surfactant with a low HLB value and a surfactant with a high HLB value according to a mass ratio of 3: 7-4: 6; s4, double-layer structure die head coating is conducted, and the ratio of Q2 to Q1 is controlled to range from 0.15 to 0.25; s5, reverse temperature gradient drying, wherein the temperature of the edge area is 10-20 DEG C lower than that of the center area. Compared with the prior art, through the synergistic effect of multiple technologies, the edge thickness ratio of the negative plate is controlled within 1.05, the energy density and rate capability of the battery are effectively improved, and the cycle life of the battery is effectively prolonged.

Description

TECHNICAL FIELD

[0001] The present application belongs to the technical field of battery pole piece preparation, and particularly relates to a negative pole piece capable of avoiding edge over-thickness and a manufacturing method thereof. BACKGROUND

[0002] With the rapid development of new energy vehicles and energy storage industries, higher requirements are put forward for the energy density, cycle life and safety of lithium ion batteries. As a key component of lithium ion batteries, the quality of the negative pole piece directly affects the performance and safety of the battery. In the preparation process of the negative pole piece, a common technical problem is that the thickness of the edge region of the coating is greater than that of the central region, forming a so-called "thick edge" phenomenon.

[0003] During the coating process of the negative pole piece, the phenomenon of thick edge and thin center often occurs. This thick edge phenomenon causes a series of technical problems: first, it causes additional mechanical stress in the edge region during the winding or stacking process of the battery, causing active material to fall off and the electrode to deform, and in severe cases, it can even pierce the separator, causing safety hazards; second, the electrochemical reaction in the thick edge region is uneven, and the lithium ion insertion / extraction is hindered, forming an electrochemically inert region, which reduces the battery capacity utilization rate; third, the thick edge causes uneven current distribution, accelerating the aging of the local region and shortening the overall service life of the battery.

[0004] The main reasons for the thick edge of the negative pole piece are as follows: first, when the slurry is extruded and sprayed from the die, the viscoelastic slurry fluid will expand, and due to the additional stress from the edge wall of the die, the slurry expansion effect is more obvious at the edge, thus causing the thick edge phenomenon; second, when the coating is dried, the drying speed is the same everywhere, but the solvent evaporates faster at the edge, so the composition changes faster at the edge. If there is no appropriate interfacial active agent or other additives in the slurry, or the surface tension of the dispersed particle suspension is greater than that of the solvent, the slurry will flow to the edge, further exacerbating the thick edge phenomenon. Specifically, as shown in FIG. 1, the solid content of the edge C of the coated area B on the foil A rises faster than the central area, the surface tension of the slurry at the edge C of the coated area B is greater than that of the central part, and the slurry flows to the edge C region, thus the pole piece after oven drying appears a "thick edge" phenomenon. Figure 1 Figure 1 In the figure, the hollow arrow points to the order of the change of the pole piece during the drying process, wherein the upward arrow in the uppermost pole piece indicates the direction of solvent evaporation during the drying process, the horizontal arrow in the middle pole piece points to the moving direction of the slurry during the drying process, and the edge C in the lowermost pole piece is a "thick edge".

[0005] ​In response to the above problems, the existing technology usually adopts the following solutions: First, reduce the coating gap, but this method has limited effect and will increase the coating pressure, which may easily lead to other coating defects; second, add surfactants or adjust the viscosity to reduce the surface tension of the slurry and inhibit the flow of the slurry to the edge during the drying process, but simply reducing the surface tension may cause the slurry to be too wettable, resulting in coating overflow or too thin thickness; third, optimize the shape of the slit gasket outlet, change the slurry flow state, and weaken the edge expansion effect, but this method requires precision processing equipment, and the die head design is complex and difficult to adjust.

[0006] The main problems with existing technical solutions are: first, most of them only focus on the optimization of a single influencing factor, such as only adjusting the rheological properties of the slurry or only improving the die structure, and lack a systematic solution; second, uniform drying conditions are usually adopted, and the important influence of the drying process on the edge thickness is not considered; third, there is insufficient consideration of the characteristics of aqueous negative electrode slurry, which is gradually replacing traditional solvent-based slurry due to environmental protection requirements. Its rheological behavior and drying characteristics are significantly different from those of traditional slurry.

[0007] Therefore, there is an urgent need to develop a systematic method for producing negative electrode sheets, which can coordinately control multiple dimensions such as slurry formulation, dispersion process, die design and drying conditions, effectively solve the thick edge problem under the aqueous negative electrode slurry system, and improve the thickness uniformity of the negative electrode sheet and battery performance. Summary of the Invention

[0008] The purpose of the present invention is to address the shortcomings of the existing technology and provide a production method for achieving precise control of the lower edge thickness of an aqueous negative electrode slurry system through pulsed ultrasonic dispersion, surface tension gradient control, double-layer structure die coating and reverse temperature gradient drying, thereby improving the quality of negative electrode sheets and battery performance.

[0009] To achieve the above object, the present invention provides the following technical solutions:

[0010] A method for manufacturing a negative electrode sheet that can avoid excessive edge thickness comprises the following steps:

[0011] S1. Preparation of negative electrode slurry: Graphite and silicon-carbon composite material were mixed in a mass ratio of (90-95):(5-10) as active materials, a conductive agent and an aqueous binder were added, and deionized water was used as a solvent to prepare a negative electrode slurry with a solid content of 45-55 wt%;

[0012] S2. Pulsed ultrasonic dispersion: The negative electrode slurry is dispersed using intermittent pulsed ultrasonic waves with an ultrasonic frequency of 40-60 kHz. The ratio of pulse working time to intermittent time is (2-3):1, and the total processing time is 15-30 minutes.

[0013] S3, surface tension gradient control: adding a composite surfactant composed of two different HLB value surfactants in the negative electrode slurry, the mass ratio of low HLB value surfactant to high HLB value surfactant is 3:7~4:6, control the surface tension of the slurry in the range of 25~32mN / m;

[0014] S4, double-layer structure die coating: using a double-layer structure die with a main chamber and an auxiliary chamber for coating, the slurry flow in the main chamber is Q1, the slurry flow in the auxiliary chamber is Q2, the ratio of Q2 / Q1 is controlled to be 0.15~0.25, the outlet width of the auxiliary chamber is 20~30% narrower than that of the main chamber;

[0015] S5, reverse temperature gradient drying: placing the coated negative electrode sheet in a drying system with a temperature gradient, so that the edge area temperature of the negative electrode sheet is 10~20℃ lower than the center area temperature, and the drying time is 3~6 minutes.

[0016] Preferably, the slurry composition in S1 is: graphite and silicon-carbon composite as active material accounting for 94~97wt% of the total solid content, conductive agent accounting for 1~3wt%, and water-based binder accounting for 2~5wt%; the water-based binder is a mixture of sodium carboxymethyl cellulose and butadiene rubber, and the mass ratio of the two is 1:2~1:4.

[0017] Preferably, the power density of the pulse ultrasonic dispersion in S2 is 300~500W / L, and the temperature is controlled in the range of 20~30℃; each pulse cycle includes 2 minutes of working time and 1 minute of intermittent time, and the slurry is stirred at a low speed of 25~40r / min during the intermittent period.

[0018] Preferably, the low HLB value surfactant in S3 is fatty alcohol polyoxyethylene ether, the HLB value is 4~8, and the addition amount is 0.3~0.5wt%; the high HLB value surfactant is polyethylene glycol or polypropylene oxide, the HLB value is 12~16, and the addition amount is 0.7~1.0wt%.

[0019] Preferably, the main chamber and auxiliary chamber of the double-layer structure die in S4 are configured as follows: the main chamber width is 300~350mm, and the height is 0.5~1.0mm; the auxiliary chamber is located on both sides of the edge of the main chamber, the width is 15~25mm, and the height is 0.3~0.6mm, the slurry in the auxiliary chamber is supplied by an independent feeding system, and the slurry viscosity in the auxiliary chamber is 20~30% lower than that in the main chamber.

[0020] Preferably, the drying system in S5 adopts zoning temperature control, the drying area is divided into two independent temperature control areas of central area and edge area, the temperature of the central area is set to 110-130 DEG C, the temperature of the edge area is set to 90-110 DEG C, and the edge area is provided with a cold air auxiliary device with a wind speed of 0.5-1.5 m / s.

[0021] Preferably, the surface temperature distribution of the negative plate is monitored in real time by using an infrared thermal imager in S5, and the heating power of each area is automatically adjusted by a closed-loop control system to maintain the set temperature gradient, and the deviation is controlled within ±2 DEG C.

[0022] Preferably, a step S1a of slurry pretreatment is added between S1 and S2, the slurry is filtered through a screen with a pore size of 5-10 um to remove large particle impurities, and is preliminarily stirred at 25-30 DEG C for 30-60 minutes to make the slurry preliminarily uniform.

[0023] Preferably, a step S2a of rheological property regulation is added between S2 and S3, 0.2-0.4 wt% of sodium carboxymethyl cellulose is added to the slurry, the thixotropic index of the slurry is controlled to be within the range of 0.4-0.6, and the thixotropic recovery rate after 10 seconds of static state is greater than 80%.

[0024] Preferably, a step S4a of thickness evaluation and quality control is added after S4, a laser displacement detection system is used to measure the thickness distribution of the negative plate, the ratio H' of the thickness of the edge area (within 5 mm from the edge) to the thickness of the central area is calculated, H' is controlled to be less than or equal to 1.05, and the thickness uniformity score is controlled to be greater than or equal to 95.

[0025] In addition, the application also provides a negative plate prepared by the method for manufacturing the negative plate capable of avoiding edge over-thickness.

[0026] Compared with the prior art, the application has at least the following beneficial effects:

[0027] 1) The application solves the problems of local overheating and uneven dispersion of traditional continuous ultrasonic dispersion technology through innovative pulse ultrasonic dispersion technology. The pulse mode makes the ultrasonic energy more uniformly distributed in the slurry, and the low-speed stirring during the intermittent period further promotes the dispersion uniformity. This dispersion method significantly improves the dispersion stability of the aqueous negative electrode slurry, makes the active material and conductive agent more uniformly distributed, reduces the existence of agglomerates, and reduces the edge thickness abnormality caused by uneven slurry from the source.

[0028] 2) The invention initiates the surface tension gradient control technology. Through the accurate proportioning (mass ratio of 3:7~4:6) of low HLB value and high HLB value surfactants, a specific surface tension gradient is formed on the surface and inside of the slurry. This gradient produces a Marangoni flow from the edge to the center during the drying process, effectively offsetting the tendency of slurry migration to the edge in the traditional drying process. Compared with the traditional method of reducing surface tension by single surfactant, this technology not only optimizes the wet spreading performance of the slurry, but also accurately controls the flow behavior during the drying process, fundamentally improving the thickness uniformity.

[0029] 3) The double-layer structure die designed by the invention realizes accurate regulation of the slurry flow in the edge area through the synergistic effect of the main chamber and the auxiliary chamber. The slurry viscosity in the auxiliary chamber is 20~30% lower than that in the main chamber, and the flow ratio is controlled at Q2 / Q1=0.15~0.25. This design enhances the flowability of the slurry in the edge area and weakens the swelling effect, thus controlling the edge thickness from the level of the die outlet. Compared with the traditional single-chamber die, this structure can specifically solve the edge swelling problem, and the parameters can be flexibly adjusted according to different slurry characteristics, thus having stronger adaptability.

[0030] 4) The invention creatively applies the reverse temperature gradient drying technology, which reverses the tendency of solvent preferentially evaporating from the edge in the traditional drying process by making the temperature in the edge area 10~20℃ lower than that in the central area. This drying method is combined with an infrared thermal imager and a closed-loop control system to realize accurate temperature control of ±2℃, ensuring a stable temperature gradient from the edge to the center. This technology effectively slows down the solvent evaporation rate in the edge area and inhibits the migration of slurry to the edge, further controlling the edge thickness during the drying stage.

[0031] 5) Through the synergistic effect of the above steps, the invention realizes excellent control effect of edge thickness ratio H'≤1.05, which is significantly better than the traditional process level. This high-uniformity negative electrode sheet reduces the mechanical stress inside the battery, improves the uniformity of electrochemical reaction, and significantly improves the energy density, rate performance, cycle life and safety of the battery, thus having important practical value. BRIEF DESCRIPTION OF DRAWINGS

[0032] Figure 1 A process diagram of the edge thickening in the coating area of the negative electrode sheet in the prior art. DETAILED DESCRIPTION

[0033] The technical solutions of the invention will be described below in conjunction with specific embodiments. Obviously, the described embodiments are only part of the embodiments of the invention, rather than all the embodiments. Based on the embodiments in the invention, all other embodiments obtained by those skilled in the art without creative labor fall within the scope of protection of the invention.

[0034] In a first aspect according to the present application, the present application provides a manufacturing method of an anode sheet capable of avoiding edge over-thickness, comprising the following steps:

[0035] S1: anode slurry preparation, mixing graphite and silicon-carbon composite material as active material according to mass ratio (90-95):(5-10), adding conductive agent and water-based binder, and taking deionized water as solvent to prepare anode slurry with solid content of 45-55wt%;

[0036] S2: pulse ultrasonic dispersion, adopting intermittent pulse ultrasonic wave to disperse the anode slurry, the ultrasonic frequency is 40-60 kHz, the ratio of pulse working time to intermittent time is (2-3):1, and the total processing time is 15-30 minutes;

[0037] S3: surface tension gradient control, adding composite surfactant composed of two surfactants with different HLB values in the anode slurry, the mass ratio of low HLB value surfactant to high HLB value surfactant is 3:7-4:6, and the surface tension of the slurry is controlled in the range of 25-32 mN / m;

[0038] S4: double-layer structure die coating, adopting double-layer structure die with main chamber and auxiliary chamber for coating, the slurry flow in the main chamber is Q1, the slurry flow in the auxiliary chamber is Q2, the ratio of Q2 / Q1 is controlled to be 0.15-0.25, and the outlet width of the auxiliary chamber is 20-30% narrower than that of the main chamber;

[0039] S5: reverse temperature gradient drying, placing the coated anode sheet in a drying system with temperature gradient, so that the temperature of the edge region of the anode sheet is 10-20℃ lower than that of the center region, and the drying time is 3-6 minutes.

[0040] In an embodiment according to the present application, the slurry in S1 has a composition of 94-97wt% of graphite and silicon-carbon composite as active material, 1-3wt% of conductive agent, and 2-5wt% of aqueous binder; the aqueous binder is a mixture of sodium carboxymethyl cellulose and styrene-butadiene rubber with a mass ratio of 1:2-1:4. The combination of graphite and silicon-carbon composite can improve the energy density and cycle performance of the negative electrode, and is the mainstream negative electrode material for current high-performance lithium-ion batteries. Graphite material provides stable structure and good electrical conductivity, while silicon-carbon composite provides high specific capacity, and the combination of the two can balance capacity and cycle stability. The active material content is controlled within the range of 94-97wt%, which is too low to reduce the battery energy density, and too high to affect the stability of the electrode structure and ion conductivity. The aqueous binder uses a mixture of sodium carboxymethyl cellulose (CMC) and styrene-butadiene rubber (SBR), CMC provides good dispersibility and adhesion, SBR provides flexibility and elasticity, and the combination of the two can significantly improve the mechanical strength and flexibility of the negative electrode sheet. The mass ratio of CMC to SBR is controlled within the range of 1:2-1:4, which is the best ratio determined through a large number of experiments, which can ensure sufficient bonding strength and maintain good flexibility, effectively preventing cracking and warping during the drying process.

[0041] In an embodiment according to the present application, the power density of the pulsed ultrasonic dispersion in S2 is 300-500W / L, and the temperature is controlled within the range of 20-30℃; each pulse cycle includes 2 minutes of working time and 1 minute of intermittent time, and the slurry is stirred at a low speed of 25-40r / min during the intermittent period. Pulsed ultrasonic dispersion is one of the key technologies of the present application, which has the following advantages compared with traditional continuous ultrasonic dispersion: first, it avoids the local overheating problem caused by continuous ultrasonic dispersion, preventing abnormal changes in slurry viscosity; second, through the working-intermittent alternating mode, the dispersion process is more uniform and thorough, reducing the "dead angle"; third, it reduces the possibility of re-aggregation of agglomerates. The power density is controlled within the range of 300-500W / L, which can provide sufficient cavitation effect to disperse the agglomerates without damaging the material structure; the temperature is controlled within the range of 20-30℃, which avoids the performance changes of the binder caused by high temperature. Each pulse cycle includes 2 minutes of working time and 1 minute of intermittent time, which is an optimized ratio that can ensure dispersion effect while avoiding material damage caused by excessive dispersion. Low-speed stirring during the intermittent period helps to homogenize the slurry, making the ultrasonic effect more uniform. Through this pulsed ultrasonic dispersion technology, the active material, conductive agent and binder in the slurry can form a uniform and stable dispersion system, significantly reducing the existence of agglomerates, laying a foundation for subsequent coating process.

[0042] In one embodiment of the present application, the low-HLB surfactant in S3 is a fatty alcohol polyoxyethylene ether with an HLB value of 4-8, added in an amount of 0.3-0.5 wt%. The high-HLB surfactant is polyethylene glycol or polyoxypropylene with an HLB value of 12-16, added in an amount of 0.7-1.0 wt%. This invention innovatively utilizes surface tension gradient control technology, unlike the traditional use of a single surfactant. The combined use of a low-HLB surfactant (fatty alcohol polyoxyethylene ether) and a high-HLB surfactant (polyethylene glycol or polyoxypropylene) can create a surface tension gradient at the slurry surface. The low-HLB surfactant is highly lipophilic and acts primarily at the interface between the slurry surface and air, significantly reducing surface tension. The high-HLB surfactant is highly hydrophilic and acts primarily at the interface between the slurry and solid particles, improving dispersion stability. This surface tension gradient plays a crucial role in the drying process: when the solvent begins to evaporate, the surfactant concentration changes, resulting in a surface tension gradient, which generates Marangoni flow from the edge to the center, effectively suppressing the tendency of the slurry to migrate toward the edges. The optimal ratio of low-HLB to high-HLB surfactants, determined experimentally to be 3:7 to 4:6, effectively suppresses thick edge formation while maintaining slurry stability. The total surfactant loading (1.0-1.5 wt%) was optimized to effectively reduce surface tension to the ideal range of 25-32 mN / m without excessively impacting the electrochemical performance of the electrode material.

[0043] In an embodiment according to the present application, the main chamber and the auxiliary chamber of the double-layer structure die in the S4 are configured as follows: the main chamber has a width of 300-350 mm and a height of 0.5-1.0 mm; the auxiliary chamber is located at the two side edges of the main chamber, has a width of 15-25 mm and a height of 0.3-0.6 mm, is fed by an independent feeding system, and the slurry in the auxiliary chamber has a viscosity that is 20-30% lower than that of the slurry in the main chamber. The double-layer structure die is one of the core technical innovations of the present application, which fundamentally changes the flow field distribution of the traditional single-chamber die. The main chamber is responsible for coating the central region, and the auxiliary chamber is specially designed to precisely control the edge region. The width (300-350 mm) of the main chamber is determined according to the actual production needs of the coating width, and the height (0.5-1.0 mm) determines the main coating thickness; the auxiliary chamber is located on both sides of the main chamber and is specially designed to regulate the edge region, and its width (15-25 mm) and height (0.3-0.6 mm) are carefully designed to accurately regulate the slurry flow of the edge region. The most critical innovation is that the viscosity of the slurry in the auxiliary chamber is 20-30% lower than that in the main chamber, which is achieved by adjusting the binder content of the slurry in the auxiliary chamber or adding a small amount of rheological modifier. The slurry with lower viscosity has better flowability in the edge region and also has a smaller degree of swelling effect, and the flow ratio between the main chamber and the auxiliary chamber is controlled within the range of Q2 / Q1=0.15-0.25, which ensures that the edge region has sufficient slurry coverage but not excessive. This special design effectively suppresses the swelling effect in the edge region in traditional coating and controls the edge thickness from the source.

[0044] In an embodiment according to the present application, the drying system in S5 adopts zoned temperature control, which divides the drying area into two independent temperature control zones, central zone and edge zone. The temperature of the central zone is set to 110-130 DEG C, and the temperature of the edge zone is set to 90-110 DEG C. At the same time, the edge zone is provided with a cold air auxiliary device with a wind speed of 0.5-1.5 m / s. The reverse temperature gradient drying is another important innovation of the present application, which is in sharp contrast to the traditional uniform temperature drying method. In the traditional drying process, the edge region of the negative plate is exposed to the heat environment on three sides, and the solvent evaporation rate is faster, which can easily lead to the intensification of the thick edge phenomenon caused by the slurry migration to the edge. The present application reverses this trend by using zoned temperature control technology to make the temperature of the edge region 10-20 DEG C lower than that of the central region, creating a drying gradient from the center to the edge. The temperature of the central zone is set to 110-130 DEG C to ensure sufficient drying rate; the temperature of the edge zone is reduced to 90-110 DEG C to significantly slow down the solvent evaporation rate in the edge region. At the same time, the cold air auxiliary device with a wind speed of 0.5-1.5 m / s is arranged in the edge region to further strengthen the temperature gradient effect. This reverse temperature gradient drying technology fundamentally changes the migration behavior of the slurry during the drying process, making the solvent evaporate from the central region first, resulting in a migration trend from the edge to the center, effectively offsetting the edge expansion effect and achieving precise control of the thickness.

[0045] In an embodiment according to the present application, the infrared thermal imager is used in S5 to monitor the surface temperature distribution of the negative plate in real time, and the closed-loop control system automatically adjusts the heating power of each region to maintain the set temperature gradient, with a deviation control within ±2 DEG C. In order to ensure the accurate control of the reverse temperature gradient, the present application adopts an advanced closed-loop control system. The infrared thermal imager captures the temperature distribution of the negative plate surface in real time, with a resolution of 0.1 DEG C, forming a complete temperature field image; the closed-loop control system receives these temperature data, automatically calculates and adjusts the heating power of each region and the wind speed of the cold air device according to the preset temperature gradient target. The system response time is not more than 2 seconds, which can quickly respond to the dynamic changes in the drying process. The temperature deviation is controlled within ±2 DEG C, ensuring the stability and consistency of the drying conditions. This precise temperature gradient control technology makes the drying process predictable and repeatable, greatly improving the stability of production and the consistency of product quality.

[0046] In an embodiment according to the present application, a step S1a of slurry pre-treatment is added between S1 and S2: the slurry is filtered through a screen with a pore size of 5-10 pm to remove large-particle impurities, and pre-stirred for 30-60 minutes at 25-30°C to make the slurry preliminarily uniform. The addition of the slurry pre-treatment step improves the reliability and stability of the entire process. Filtration through a screen with a pore size of 5-10 pm can effectively remove large-particle impurities that may exist in the slurry, such as agglomerated active materials, lumps of binder, or impurities mixed in the production environment, avoiding defects such as scratches and uneven thickness caused by these large particles in the subsequent coating process. Pre-stirring is carried out under mild conditions (25-30°C) to preliminarily mix the components of the slurry uniformly, creating good conditions for subsequent pulse ultrasonic dispersion. The stirring time is controlled within the range of 30-60 minutes, and too short a time makes it difficult to achieve uniformity, and too long a time may cause some components to settle or stratify. This pre-treatment step, although simple, is of great significance to improving the quality of the final product, especially for complex negative electrode slurry systems containing multiple components.

[0047] In an embodiment according to the present application, a step S2a of rheological property adjustment is added between S2 and S3: 0.2-0.4wt% of sodium carboxymethyl cellulose is added to the slurry to control the thixotropic index of the slurry within the range of 0.4-0.6, and the thixotropic recovery rate after 10 seconds of stillness is greater than 80%. The rheological property adjustment step is designed for the special properties of the aqueous negative electrode slurry and is an important supplement to the basic process. The rheological behavior of the aqueous negative electrode slurry is more complex than that of traditional organic solvent-based slurries, directly affecting the coating quality and drying uniformity. Adding an appropriate amount of sodium carboxymethyl cellulose (CMC) not only serves as a binder, but more importantly as a rheological modifier, which can significantly improve the thixotropy of the slurry. Thixotropy refers to the property of the slurry to reduce viscosity under the action of shear force and recover viscosity after removal of the shear force. The thixotropic index is controlled within the range of 0.4-0.6, indicating that the slurry has sufficient fluidity for coating and sufficient structure to prevent excessive flow leveling after coating; the thixotropic recovery rate after 10 seconds of stillness is greater than 80%, ensuring that the slurry quickly recovers a certain degree of structure after coating, limiting the free flow of the slurry and effectively suppressing the accumulation of slurry in the edge area. This precise rheological property control is an important auxiliary means to suppress the thick edge phenomenon, and in combination with other technical measures, further improves the coating uniformity.

[0048] In an embodiment according to the present application, the S4 post-increase step S4a: thickness evaluation and quality control, uses a laser displacement detection system to measure the thickness distribution of the negative plate, calculates the ratio H' of the thickness of the edge area (within 5 mm from the edge) to the thickness of the central area, controls H'≤1.05, and controls the thickness uniformity score to be ≥95 points. The thickness evaluation and quality control step establishes a scientific evaluation system, providing an objective basis for process optimization. The laser displacement detection system has a micron-level precision, and can accurately capture the thickness variation of the negative plate in the transverse direction. The edge thickness ratio H' is defined as the ratio of the average thickness of the edge area (within 5 mm from the edge) to the average thickness of the central area, and is a key indicator for measuring the severity of thick edge phenomenon. The strict standard of H'≤1.05 means that the thickness of the edge area does not exceed the center area by more than 5%, which is much better than the traditional process level of 10-15%. The thickness uniformity score is based on the standard deviation calculation of the full plate thickness, and a score of ≥95 points indicates that the thickness distribution is extremely uniform. This strict quality control system not only provides clear indicators for production, but also provides quantitative basis for continuous improvement. Through real-time monitoring and evaluation, process parameters can be adjusted in time to ensure that product quality is stable at a high level.

[0049] In an embodiment according to the present application, the pulsed ultrasonic dispersion device includes an ultrasonic generator, a temperature control system, and a pulse controller. The ultrasonic generator generates ultrasonic waves of 40-60 kHz, which are transmitted into the slurry through a probe; the temperature control system consists of a cooling circulation device and a temperature sensor, which monitors the slurry temperature in real time and keeps it within the range of 20-30°C; the pulse controller controls the switching of the ultrasonic waves according to a working-interval time ratio of (2-3):1, and starts a low-speed stirring device during the interval, with a stirring speed of 25-40 r / min.

[0050] In an embodiment according to the present application, the double-layer structure die mainly consists of a main chamber and an auxiliary chamber. The main chamber has a width of 300-350 mm and a height of 0.5-1.0 mm, and is used for coating the central area; the auxiliary chamber is located on both sides of the main chamber, has a width of 15-25 mm and a height of 0.3-0.6 mm, and is specially responsible for the control of the edge area. The two chambers are fed by independent feeding systems, and the slurry in the auxiliary chamber has a viscosity that is 20-30% lower than that in the main chamber by adjusting the formula. The main chamber flow rate Q1 and the auxiliary chamber flow rate Q2 are adjusted by a precision flow control system, keeping the Q2 / Q1 ratio within the range of 0.15-0.25.

[0051] In an embodiment according to the present application, the reverse temperature gradient drying system comprises a central heating zone and an edge temperature control zone, both of which are separated by a heat insulation material and independently controlled. The temperature of the central zone is set to 110-130℃, and the temperature of the edge zone is set to 90-110℃, forming a temperature gradient decreasing from the center to the edge. An infrared thermal imager monitors the surface temperature distribution of the negative plate in real time, and the data is transmitted to a closed-loop control system to automatically adjust the heating power of each zone. The edge zone is also provided with a cold air auxiliary device, and the wind speed can be adjusted within the range of 0.5-1.5m / s, further strengthening the effect of the temperature gradient.

[0052] In a second aspect according to the present application, the present application also provides a negative plate prepared by the method for preparing a negative plate with avoidable edge over-thickness described in any of the above paragraphs.

[0053] The implementation and advantages of the present application will be further described below in combination with specific embodiments.

[0054] Example 1

[0055] S1: Preparation of negative electrode slurry, mix natural graphite and silicon-carbon composite material according to a mass ratio of 95:5 as active material (totaling 96wt%), add conductive carbon black (2wt%) and water-based binder (2wt%, mass ratio of CMC to SBR is 1:3), and deionized water as solvent, to prepare a negative electrode slurry with a solid content of 50wt%;

[0056] S1a: Pre-treatment of slurry, filter the slurry through a nylon screen with a pore size of 8μm, and pre-stir for 45 minutes at 28℃ until uniform;

[0057] S2: Pulse ultrasonic dispersion, treat the slurry with an ultrasonic power density of 400W / L, ultrasonic frequency of 50kHz, each cycle including 2 minutes of working time and 1 minute of intermittent time, low-speed stirring of 30r / min during the intermittent period, temperature control at 25℃, total treatment time of 24 minutes (a total of 8 cycles);

[0058] S2a: Rheological property adjustment, add 0.3wt% of sodium carboxymethyl cellulose to the slurry, and measure the thixotropic index of the slurry to be 0.45, and the thixotropic recovery rate after 10 seconds of static state to be 88%;

[0059] S3: Surface tension gradient control, add 0.4wt% of fatty alcohol polyoxyethylene ether (HLB value of 6) and 0.8wt% of polyethylene glycol (HLB value of 14) to the negative electrode slurry, mass ratio of 1:2, and measure the surface tension of the slurry to be 30mN / m;

[0060] S4: Double-layer structure die coating, using a double-layer die with a main chamber width of 330 mm and a height of 0.8 mm, and an auxiliary chamber width of 20 mm and a height of 0.5 mm, the auxiliary chamber slurry viscosity is 25% lower than the main chamber (achieved by reducing 0.5wt% SBR), control the main chamber flow rate Q1 to be 100 mL / min, the auxiliary chamber flow rate Q2 to be 20 mL / min, Q2 / Q1=0.2;

[0061] S4a: Thickness evaluation, using a laser displacement detection system to measure the thickness distribution of the negative electrode sheet, the average thickness of the edge region (within 5 mm from the edge) is 63.2 μm, the average thickness of the center region is 61.8 μm, the edge thickness ratio H' = 1.023, the thickness uniformity score is 97 points;

[0062] S5: Reverse temperature gradient drying, using a partition temperature control drying system, the central zone temperature is set to 120°C, the edge zone temperature is set to 100°C, the edge region is configured with a cold air auxiliary device with a wind speed of 1.0 m / s, an infrared thermal imager is used to monitor the temperature distribution in real time and adjust the heating power through a closed loop control system, the drying time is 4.5 minutes.

[0063] Example 2

[0064] S1: Negative electrode slurry preparation, mixing artificial graphite and silicon-carbon composite material according to a mass ratio of 94:6 as active material (totaling 95wt%), adding acetylene black (1.5wt%) and conductive carbon nanotubes (0.5wt%) as conductive agent, adding water-based binder (3wt%, CMC to SBR mass ratio is 1:2), and using deionized water as solvent to prepare a negative electrode slurry with a solid content of 48wt%;

[0065] S1a: Slurry pretreatment, the slurry is filtered through a stainless steel screen with a pore size of 6 μm, and pre-stirred for 50 minutes at 26°C until uniform;

[0066] S2: Pulse ultrasonic dispersion, the slurry is treated with ultrasonic waves with a power density of 450 W / L, the ultrasonic frequency is 45 kHz, each cycle includes 2 minutes of working time and 1 minute of intermittent time, low-speed stirring at 35 r / min is carried out during the intermittent period, the temperature is controlled at 23°C, the total treatment time is 21 minutes (a total of 7 cycles);

[0067] S2a: Rheological property adjustment, 0.25wt% of sodium carboxymethyl cellulose is added to the slurry, the thixotropic index of the slurry is measured to be 0.5, and the thixotropic recovery rate after 10 seconds of static state is 85%;

[0068] S3: Surface tension gradient control, 0.35wt% of fatty alcohol polyoxyethylene ether (HLB value of 7) and 0.75wt% of polyoxypropylene (HLB value of 13) were added to the negative electrode slurry, the mass ratio of the two was 7:15, and the surface tension of the slurry was measured to be 28 mN / m;

[0069] S4: Double-layer structure die coating, a double-layer die with a main chamber width of 320 mm and a height of 0.7 mm and an auxiliary chamber width of 18 mm and a height of 0.4 mm was used for coating, the auxiliary chamber slurry viscosity was 22% lower than the main chamber (achieved by adding 0.1wt% rheological modifier), the main chamber flow rate Q1 was controlled at 90 mL / min, the auxiliary chamber flow rate Q2 was 18 mL / min, and Q2 / Q1=0.2;

[0070] S4a: Thickness evaluation, the thickness distribution of the negative electrode sheet was measured using a laser displacement detection system, the average thickness of the edge region (within 5 mm from the edge) was 58.7 μm, the average thickness of the center region was 57.8 μm, the edge thickness ratio H' was 1.016, and the thickness uniformity score was 98 points;

[0071] S5: Reverse temperature gradient drying, a partitioned temperature control drying system was used, the central zone temperature was set to 125°C, the edge zone temperature was set to 105°C, the edge region was equipped with a cold air auxiliary device with a wind speed of 1.2 m / s, an infrared thermal imager was used to monitor the temperature distribution in real time and adjust the heating power through a closed-loop control system, and the drying time was 4 minutes.

[0072] Example 3

[0073] S1: Negative electrode slurry preparation, natural graphite, artificial graphite and silicon-carbon composite material were mixed in a mass ratio of 45:50:5 as active material (totaling 97wt%), conductive carbon black (1wt%) and water-based binder (2wt%, CMC to SBR mass ratio of 1:4) were added, deionized water was used as solvent, and a negative electrode slurry with a solid content of 52wt% was prepared;

[0074] S1a: Slurry pretreatment, the slurry was filtered through a polypropylene screen with a pore size of 5 μm, and pre-stirring was carried out at 30°C for 40 minutes until uniform;

[0075] S2: Pulse ultrasonic dispersion, the slurry was treated with ultrasonic waves with a power density of 350 W / L, the ultrasonic frequency was 55 kHz, each cycle included 2 minutes of working time and 1 minute of intermittent time, low-speed stirring at 28 r / min was carried out during the intermittent period, the temperature was controlled at 22°C, and the total treatment time was 27 minutes (a total of 9 cycles);

[0076] S2a: rheology adjustment, 0.35wt% of sodium carboxymethyl cellulose is added to the slurry, the thixotropic index of the slurry is measured to be 0.42, and the thixotropic recovery rate after 10 seconds of rest is 92%;

[0077] S3: surface tension gradient control, 0.45wt% of fatty alcohol polyoxyethylene ether (HLB value is 5) and 0.75wt% of polyethylene glycol (HLB value is 15) are added to the negative electrode slurry, the mass ratio of the two is 3:5, and the surface tension of the slurry is measured to be 26mN / m;

[0078] S4: double-layer structure die coating, a double-layer die with a main chamber width of 340mm and a height of 0.9mm and an auxiliary chamber width of 22mm and a height of 0.55mm is used for coating, the auxiliary chamber slurry viscosity is 28% lower than the main chamber (achieved by adding 0.15wt% rheology modifier), the main chamber flow rate Q1 is controlled to be 110mL / min, the auxiliary chamber flow rate Q2 is 22mL / min, and Q2 / Q1=0.2;

[0079] S4a: thickness evaluation, the thickness distribution of the negative electrode sheet is measured using a laser displacement detection system, the average thickness of the edge region (within 5mm from the edge) is measured to be 72.1μm, the average thickness of the center region is 70.5μm, the edge thickness ratio H' is 1.023, and the thickness uniformity score is 97;

[0080] S5: reverse temperature gradient drying, a partition temperature control drying system is used, the central zone temperature is set to 115℃, the edge zone temperature is set to 95℃, the edge region is configured with a cold air auxiliary device with a wind speed of 0.8m / s, an infrared thermal imager is used to monitor the temperature distribution in real time and adjust the heating power through a closed loop control system, and the drying time is 5 minutes.

[0081] Comparative Example 1

[0082] A negative electrode sheet is prepared using a traditional single-layer die and a uniform drying process:

[0083] S1: natural graphite and silicon-carbon composite material are mixed in a mass ratio of 95:5 as active material (96wt%), conductive carbon black (2wt%) and water-based binder (2wt%, CMC to SBR mass ratio is 1:3) are added, deionized water is used as solvent, and a negative electrode slurry with a solid content of 50wt% is prepared;

[0084] S2: the slurry is dispersed by conventional stirring, the stirring speed is 500r / min, the temperature is room temperature, and the time is 60 minutes;

[0085] S3: 0.8wt% of a single surfactant (polyethylene glycol) is added, and the surface tension of the slurry is measured to be 35mN / m;

[0086] S4: Use traditional single-layer die coating, die width is 330mm, height is 0.8mm;

[0087] S5: conventional uniform drying is adopted, the drying temperature is constant at 120°C, and the drying time is 4.5 minutes;

[0088] S6: Thickness evaluation: the average thickness of the edge area was measured to be 76.4 μm, the average thickness of the center area was 62.5 μm, the edge thickness ratio H′=1.222, and the thickness uniformity score was 68 points.

[0089] Comparative Example 2

[0090] The negative electrode sheet was prepared by using only surfactant optimization method:

[0091] S1: Natural graphite and silicon-carbon composite material were mixed in a mass ratio of 95:5 as the active material (96 wt%), conductive carbon black (2 wt%) and aqueous binder (2 wt%) were added, and deionized water was used as the solvent to prepare a negative electrode slurry with a solid content of 50 wt%;

[0092] S2: The slurry was dispersed by conventional stirring at a speed of 500 r / min, room temperature, and time of 60 minutes;

[0093] S3: 1.2 wt% of a composite surfactant (0.4 wt% of fatty alcohol polyoxyethylene ether and 0.8 wt% of polyethylene glycol) was added, and the surface tension of the slurry was measured to be 30 mN / m;

[0094] S4: Use traditional single-layer die coating, die width is 330mm, height is 0.8mm;

[0095] S5: conventional uniform drying is adopted, the drying temperature is constant at 120°C, and the drying time is 4.5 minutes;

[0096] S6: Thickness evaluation: the average thickness of the edge area was measured to be 71.3 μm, the average thickness of the center area was 63.1 μm, the edge thickness ratio H′=1.130, and the thickness uniformity score was 79 points.

[0097] Comparative Example 3

[0098] The negative electrode sheet is prepared by only using pulsed ultrasonic dispersion and a double-layer die head:

[0099] S1: Natural graphite and silicon-carbon composite material were mixed in a mass ratio of 95:5 as the active material (96 wt%), conductive carbon black (2 wt%) and aqueous binder (2 wt%) were added, and deionized water was used as the solvent to prepare a negative electrode slurry with a solid content of 50 wt%;

[0100] S2: The slurry was dispersed by pulse ultrasonic dispersion treatment, the ultrasonic frequency was 50 kHz, the ratio of working time to intermittent time was 2:1, and the total treatment time was 24 minutes;

[0101] S3: 0.8wt% of a single surfactant (polyethylene glycol) was added, and the surface tension of the slurry was measured to be 35 mN / m;

[0102] S4: The slurry was coated by using a double-layer structure die, the main chamber width was 330 mm, the height was 0.8 mm, the auxiliary chamber width was 20 mm, the height was 0.5 mm, the auxiliary chamber slurry viscosity was 25% lower than that of the main chamber, and Q2 / Q1 was controlled to be 0.2;

[0103] S5: The slurry was dried by using a conventional uniform drying method, the drying temperature was constant at 120°C, and the drying time was 4.5 minutes;

[0104] S6: Thickness evaluation, the average thickness of the edge region was measured to be 67.8 μm, the average thickness of the center region was 62.7 μm, the edge thickness ratio H' was 1.081, and the thickness uniformity score was 87.

[0105] Comparative Example 4

[0106] The negative electrode sheet was prepared by using only the reverse temperature gradient drying method:

[0107] S1: The natural graphite and silicon-carbon composite material were mixed according to a mass ratio of 95:5 as the active material (96wt%), conductive carbon black (2wt%) and water-based binder (2wt%) were added, deionized water was used as the solvent, and a negative electrode slurry with a solid content of 50wt% was prepared;

[0108] S2: The slurry was dispersed by using a conventional stirring method, the stirring speed was 500 r / min, the temperature was room temperature, and the time was 60 minutes;

[0109] S3: 0.8wt% of a single surfactant (polyethylene glycol) was added, and the surface tension of the slurry was measured to be 35 mN / m;

[0110] S4: The slurry was coated by using a traditional single-layer die, the die width was 330 mm, and the height was 0.8 mm;

[0111] S5: The slurry was dried by using a reverse temperature gradient drying method, the central zone temperature was 120°C, the edge zone temperature was 100°C, and the drying time was 4.5 minutes;

[0112] S6: Thickness evaluation, the average thickness of the edge region was measured to be 69.5 μm, the average thickness of the center region was 63.2 μm, the edge thickness ratio H' was 1.100, and the thickness uniformity score was 82.

[0113] The negative electrode sheets prepared in the above examples and comparative examples were respectively cut into 5.0 cm x 8.0 cm, and assembled into soft pack batteries with commercial NCM811 positive electrode sheets, Celgard 2325 separator, and 1.0 M LiPF6 EC / DMC / EMC (volume ratio of 1:1:1) electrolyte, and packaged in an argon glove box.

[0114] The prepared negative electrode sheets and batteries were subjected to the following performance tests, and the test results are shown in Tables 1 and 2.

[0115] 1. Negative electrode sheet thickness distribution measurement method: Keyence LK-G5000 series laser displacement measurement system was used to measure a point every 2 mm along the width direction of the negative electrode sheet, and 15 lines were measured for each sample, and the average thickness and ratio of the edge region (within 5 mm from the edge) and the central region were calculated.

[0116] 2. Thickness uniformity score calculation method: According to the thickness standard deviation calculation, the score = 100 - 25 x (thickness standard deviation / average thickness), the smaller the difference, the higher the score, full score 100, the difference rate exceeds 4%, the score is 0.

[0117] 3. Negative electrode sheet bonding strength test method: 180° peeling method was used, Instron 5967 material testing machine was used, and the peeling speed was 10 mm / min, and the peeling force per unit width was calculated.

[0118] 4. Battery capacity test method: New battery test system was used, and the first charge-discharge test was carried out at 25°C with 0.1C rate, and the battery capacity and first charge-discharge efficiency were calculated.

[0119] 5. Rate performance test method: Discharge at 0.2C, 0.5C, 1C, 2C, and 5C rates at 25°C, respectively, and calculate the capacity retention rate at each rate (relative to 0.2C capacity).

[0120] 6. Cycle performance test method: Charge-discharge cycle at 1C rate for 500 times at 25°C, measure the capacity every 50 times, and calculate the capacity retention rate after 500 cycles.

[0121] 7. High and low temperature performance test method: 1C rate discharge test was carried out at 45°C and -10°C, respectively, and the capacity retention rate (relative to 1C capacity at 25°C) was calculated.

[0122] Table 1: Negative electrode sheet performance test results

[0123]

[0124] Table 2: Battery performance test results

[0125]

[0126] Wherein, the test results are analyzed as follows:

[0127] 1. Comparative analysis between examples and comparative examples:

[0128] (1) Edge thickness uniformity: the edge thickness ratios H' of Examples 1-3 are 1.023, 1.016 and 1.023 respectively, all controlled within 1.05, and the uniformity scores reach 97-98 points, significantly better than Comparative Examples 1-4. This fully proves the significant effect of the multi-technology synergistic control method of the application on suppressing edge over-thickness. Especially compared with Comparative Examples 2-4 which simply use a certain technology, the advantages of the combined strategy of the application are more obvious.

[0129] (2) Bonding strength: the bonding strengths of Examples 1-3 are 15.6, 16.2 and 15.8 N / cm respectively, which are significantly higher than Comparative Examples 1-4. This is mainly due to the pulse ultrasonic dispersion technology which makes the slurry component distribution more uniform, and the surface tension gradient control which makes the slurry and current collector bond more firmly. High bonding strength means that the active material is not easy to fall off, which is beneficial to improve the cycle stability and safety of the battery.

[0130] (3) Compaction density: the compaction densities of Examples 1-3 are in the range of 1.58-1.65 g / cm³, all higher than Comparative Examples 1-4. Uniform thickness distribution makes the negative electrode sheet more uniform in stress during compaction, reduces the invalid voids between active materials, and improves the volume energy density of the electrode.

[0131] (4) Battery performance: the batteries prepared by Examples 1-3 are significantly better than Comparative Examples 1-4 in terms of first charge-discharge efficiency, high-rate performance, cycle life and high-low temperature performance. Especially after 5C high-rate discharge and 500 times of long cycle, the capacity retention rates of Examples are significantly higher than those of Comparative Examples, indicating that the negative electrode sheet with uniform thickness can significantly improve the lithium ion transmission efficiency and electrode structure stability.

[0132] 2. Comparative analysis between examples:

[0133] (1) Example 2 performs best in many indicators, especially edge thickness uniformity (H' = 1.016), battery rate performance (87.2%) and cycle life (91.5%). This is mainly due to the optimized slurry formulation (graphite, silicon-carbon, double conductive agent system) and more precise process parameter control, such as lower slurry surface tension (28 mN / m) and more optimal auxiliary chamber design.

[0134] (2) Example 1 and Example 3, although in different parameter combinations, both achieved excellent edge thickness control (H' = 1.023) and excellent battery performance, indicating that the method of the present application has good adaptability and stability, and can be applied to different anode material systems and formula conditions.

[0135] 3. Process parameter influence analysis:

[0136] (1) Influence of pulsed ultrasonic dispersion: As can be seen from the comparison of Comparative Example 1 and Comparative Example 3, the use of pulsed ultrasonic dispersion technology reduces the edge thickness ratio from 1.222 to 1.081, indicating that uniformly dispersed slurry helps to reduce edge over-thickness. This is mainly because pulsed ultrasound destroys agglomerates, making the slurry rheology more uniform and reducing unstable flow during coating.

[0137] (2) Influence of surface tension gradient control: Comparative Example 2 uses a combination of surfactants to reduce the edge thickness ratio to 1.130, which is significantly improved compared to 1.222 of Comparative Example 1. This demonstrates the role of surface tension gradient in regulating the migration behavior of the slurry during drying, and the combination of low HLB and high HLB surfactants can form a Marangoni flow from the edge to the center during drying.

[0138] (3) Influence of double-layer structure die: The edge thickness ratio of Comparative Example 3 (1.081) is significantly lower than that of Comparative Example 1 (1.222) and Comparative Example 2 (1.130), indicating that the double-layer structure die can control the slurry flow behavior in the edge region from the source by the synergistic effect of the main and auxiliary chambers, and is an effective means to suppress thick edges.

[0139] (4) Influence of reverse temperature gradient drying: The edge thickness ratio of Comparative Example 4 is 1.100, which is better than that of Comparative Example 1 (1.222) and Comparative Example 2 (1.130), but not as good as that of Comparative Example 3 (1.081), indicating that the influence of drying conditions on edge thickness is important, but may not be as significant as the influence of die structure. However, when reverse temperature gradient drying is used in combination with other technologies, a significant synergistic effect can be achieved, as shown in Examples 1-3.

[0140] (5) Multi-technology synergistic effect: When each comparative example uses a single technology improvement, the lowest edge thickness ratio can only reach 1.081 (Comparative Example 3), while Examples 1-3, which combine all the technical measures, can reduce the edge thickness ratio to 1.016-1.023, indicating that there is a significant synergistic effect between the technical points of the present application, and the combined effect is much better than the application of a single technology.

[0141] 4. Performance improvement mechanism analysis:

[0142] (1) The reason for the improvement of the first charge-discharge efficiency: The uniform thickness of the negative electrode sheet makes the electrolyte infiltration more uniform, reducing the unevenness of the SEI film formation, and reducing the loss of invalid lithium. The first charge-discharge efficiency of Example 2 is 93.1%, which is significantly higher than 86.5% of Comparative Example 1.

[0143] (2) The reason for the improvement of the rate performance: The uniform thickness of the negative electrode sheet means that the lithium ions have similar diffusion path lengths throughout the electrode, reducing local polarization and improving the rapid charge-discharge capacity of the electrode. The capacity retention rate of Example 2 at 5C rate is 87.2%, which is much higher than 68.3% of Comparative Example 1.

[0144] (3) The reason for the improvement of the cycle life: The uniform edge thickness of the negative electrode sheet reduces local stress concentration and reduces uneven volume expansion during charging and discharging, reducing the shedding of active materials and the degradation of the electrode structure. The capacity retention rate of Example 2 after 500 cycles is 91.5%, while that of Comparative Example 1 is only 74.6%.

[0145] (4) The reason for the improvement of high and low temperature performance: The uniform electrode structure means that the electrolyte distribution and ion transport path are more consistent, with less performance degradation under extreme temperature conditions. The capacity retention rates of Examples 1-3 at 45°C and -10°C are significantly higher than those of Comparative Examples 1-4, indicating that the negative electrode sheet prepared by the method of the present application has better temperature adaptability.

[0146] 5. Theoretical analysis of process parameter optimization:

[0147] (1) The 2:1 working-intermittent time ratio of pulse ultrasonic dispersion is based on the balance of cavitation effect and thermal effect. The cavitation effect produced during ultrasonic working can effectively disperse the agglomerates, but at the same time, local heat is generated; during the intermittent period, the heat is diffused to prevent local overheating from causing slurry deterioration. This alternating mode can achieve more uniform dispersion effect without damaging the material structure.

[0148] (2) The surface tension gradient control technique utilizes the principle of the Marangoni effect. When the concentration distribution of two surfactants changes during drying, a surface tension gradient will form on the liquid surface, causing the liquid to flow from the low surface tension area to the high surface tension area. The 3:7~4:6 ratio of low HLB to high HLB surfactants can produce a flow trend from the edge to the center during drying, offsetting the tendency of the slurry to migrate to the edge during natural drying.

[0149] (3) The design of the double-layer structure die is based on fluid mechanics calculation and experimental optimization. The viscosity of the slurry in the auxiliary chamber is 20-30% lower than that in the main chamber, which makes the slurry in the edge area more flowable and reduces the edge swelling caused by viscous resistance; the Q2 / Q1 ratio is controlled in the range of 0.15-0.25, which ensures that the edge area has enough slurry coverage, but not too much to cause accumulation.

[0150] (4) The center area temperature is 10-20℃ higher than the edge area temperature in the reverse temperature gradient drying technology, and this temperature difference is calculated based on the evaporation kinetics of water in the water-based slurry. A too small temperature difference has no obvious effect, and a too large temperature difference may cause uneven drying or excessive drying and cracking in the central area.

[0151] In summary, the negative electrode sheet manufacturing method provided by the present application can avoid edge over-thickness. Through the synergistic effect of pulse ultrasonic dispersion, surface tension gradient control, double-layer structure die coating and reverse temperature gradient drying technology, the edge thickness ratio of the negative electrode sheet is successfully controlled to be less than 1.05, which is significantly better than the traditional process level. Such high-uniformity negative electrode sheet not only improves the energy density and rate performance of the battery, but also improves the cycle life and high-low temperature adaptability, which has important practical application value.

[0152] The materials preparation, battery performance testing and evaluation methods involved are conventional techniques in the art, and those skilled in the art can implement them without further description. The content protected by the present application does not involve improvements to the basic materials and testing methods.

[0153] Based on the disclosure and teaching of the above description, those skilled in the art of the present application can also make changes and modifications to the above embodiments. Therefore, the present application is not limited to the specific embodiments described above, and any obvious improvements, replacements or modifications made by those skilled in the art based on the present application all fall within the scope of protection of the present application. In addition, although some specific terms are used in the present specification, these terms are only for convenience of description and do not constitute any limitation on the present application.

Claims

1. A method for manufacturing a negative electrode sheet that can avoid excessive edge thickness, characterized in that: The following steps are involved: S1. Preparation of negative electrode slurry: Graphite and silicon-carbon composite material were mixed in a mass ratio of (90-95):(5-10) as active materials, a conductive agent and an aqueous binder were added, and deionized water was used as a solvent to prepare a negative electrode slurry with a solid content of 45-55 wt%; S2. Pulsed ultrasonic dispersion: The negative electrode slurry is dispersed using intermittent pulsed ultrasonic waves with an ultrasonic frequency of 40-60 kHz. The ratio of pulse working time to intermittent time is (2-3):1, and the total processing time is 15-30 minutes. S3. Surface tension gradient control: Add a composite surfactant composed of two surfactants with different HLB values ​​to the negative electrode slurry. The mass ratio of the low HLB value surfactant to the high HLB value surfactant is 3:7~4:

6. Control the slurry surface tension within the range of 25~32mN / m. S4. Double-layer die coating: A double-layer die with a main chamber and an auxiliary chamber is used for coating. The slurry flow rate in the main chamber is Q1, and the slurry flow rate in the auxiliary chamber is Q2. The ratio of Q2 / Q1 is controlled to be 0.15-0.

25. The outlet width of the auxiliary chamber is 20-30% narrower than the outlet width of the main chamber. S5. Reverse temperature gradient drying: Place the coated negative electrode sheet in a drying system with a temperature gradient, so that the temperature of the edge area of ​​the negative electrode sheet is 10~20℃ lower than the temperature of the center area, and the drying time is 3~6 minutes.

2. The method for manufacturing a negative electrode sheet capable of avoiding excessive edge thickness according to claim 1, characterized in that: The slurry composition in S1 is as follows: graphite and silicon-carbon composite materials as active materials account for 94-97 wt% of the total solid content, conductive agent accounts for 1-3 wt%, and aqueous binder accounts for 2-5 wt%; the aqueous binder is a mixture of sodium carboxymethyl cellulose and styrene-butadiene rubber, and the mass ratio of the two is 1:2-1:

4.

3. The method for manufacturing a negative electrode sheet capable of avoiding excessive edge thickness according to claim 1, characterized in that: The power density of the pulsed ultrasonic dispersion in S2 is 300-500 W / L, and the temperature is controlled in the range of 20-30° C.; each pulse cycle includes 2 minutes of working time and 1 minute of rest time, and the slurry is stirred at a low speed of 25-40 r / min during the rest period.

4. The method for manufacturing a negative electrode sheet capable of avoiding excessive edge thickness according to claim 1, characterized in that: The low HLB surfactant in S3 is fatty alcohol polyoxyethylene ether, with an HLB value of 4-8, and the addition amount is 0.3-0.5 wt %; the high HLB surfactant is polyethylene glycol or polyoxypropylene, with an HLB value of 12-16, and the addition amount is 0.7-1.0 wt %.

5. The method for manufacturing a negative electrode sheet capable of avoiding excessive edge thickness according to claim 1, characterized in that: The main chamber and auxiliary chamber of the double-layer structure die head in the S4 are constructed as follows: the main chamber has a width of 300-350 mm and a height of 0.5-1.0 mm; the auxiliary chambers are located on both sides of the main chamber, with a width of 15-25 mm and a height of 0.3-0.6 mm. The auxiliary chambers are fed through an independent feeding system, and the slurry viscosity in the auxiliary chambers is 20-30% lower than that in the main chamber.

6. The method for manufacturing a negative electrode sheet capable of avoiding excessive edge thickness according to claim 1, characterized in that: The S5 adopts a zoned temperature-controlled drying system, dividing the drying area into two independent temperature-controlled areas: a central area and an edge area. The temperature of the central area is set to 110-130°C, and the temperature of the edge area is set to 90-110°C. At the same time, a cold air auxiliary device with a wind speed of 0.5-1.5 m / s is provided in the edge area.

7. The method for manufacturing a negative electrode sheet capable of avoiding excessive edge thickness according to claim 6, characterized in that: In the S5, an infrared thermal imager is used to monitor the surface temperature distribution of the negative electrode in real time, and the heating power of each area is automatically adjusted through a closed-loop control system to maintain the set temperature gradient, with the deviation controlled within the range of ±2°C.

8. The method for manufacturing a negative electrode sheet capable of avoiding excessive edge thickness according to claim 1, characterized in that: Add step S1a between S1 and S2: slurry pretreatment, filter the slurry through a sieve with a pore size of 5-10 μm to remove large particles of impurities, and pre-stir at 25-30°C for 30-60 minutes to make the slurry initially uniform.

9. The method for manufacturing a negative electrode sheet capable of avoiding excessive edge thickness according to claim 1, characterized in that: Add step S2a between S2 and S3: rheological control, add 0.2-0.4wt% sodium carboxymethyl cellulose to the slurry, control the thixotropic index of the slurry within the range of 0.4-0.6, and the thixotropic recovery rate after standing for 10 seconds is greater than 80%.

10. A negative electrode sheet, characterized in that: The negative electrode sheet is manufactured by the method for manufacturing a negative electrode sheet capable of avoiding excessive edge thickness as described in any one of claims 1 to 9.

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