Processing method for preventing edge of positive plate from being too thick and positive plate

Abstract

The invention discloses a processing method for preventing the edge of a positive plate from being too thick and the positive plate, and the processing method comprises the following steps: S1, slurry pretreatment: filtering positive slurry through a precise filter screen, adding a nonionic surfactant, and uniformly stirring; s2, slurry rheology regulation and control: carrying out shear pretreatment on the slurry until the shear viscosity is 1200-1800 mPa.s; s3, a die head gasket is optimized, specifically, an inwards-concave wedge-shaped gasket is adopted, the edge thickness of the gasket is smaller than that of a central area, and a fluid resistance gradient field is formed; s4, coating is controlled through pressure pulsation, specifically, pressure fluctuation in a die head is monitored in real time through a pressure feedback system, and the extrusion speed is dynamically adjusted according to the monitoring result; and S5, differential drying treatment: enabling the coated positive plate to pass through a multi-section drying system, wherein the drying temperature of an edge area is 15-25 DEG C lower than that of a central area. The processing method provided by the invention can effectively prevent the problem that the edge of the positive plate is too thick, and improves the performance and safety of the battery.

Description

TECHNICAL FIELD

[0001] The present application belongs to the technical field of battery pole piece preparation, and particularly relates to a processing method for preventing the edge of a positive pole piece from being too thick and a positive pole piece. BACKGROUND

[0002] With the rapid development of new energy vehicles and portable electronic devices, 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 coating quality of the positive pole piece directly affects the performance and safety of the battery. During the coating process of the positive pole piece, a common problem is that the thickness of the edge region of the coating is greater than that of the central region (as shown in FIG. 1), that is, the so-called "thick edge" phenomenon. Figure 1

[0003] The main problems caused by the thick edge phenomenon include: first, the edge region will generate additional stress during the winding or stacking process of the positive pole piece, which will cause the active material to fall off, the electrode to deform, and even the separator to be punctured, thereby causing a safety accident; second, the lithium ion diffusion in the over-thick edge region is blocked, forming an electrochemically inactive area, which reduces the battery capacity utilization rate; third, the uneven thickness of the edge leads to uneven current distribution, which accelerates the local attenuation of the battery and shortens the cycle life of the battery.

[0004] In the prior art, there are two main reasons for the occurrence of the thick edge: first, when the die extrusion is sprayed, the viscoelastic slurry fluid will expand, and due to the additional stress action of the edge wall of the die, the slurry expansion effect at the edge is more obvious, which leads to the thick edge phenomenon; second, when the coating is dried, the solvent evaporates faster at the edge, and when there is no appropriate interfacial active agent or the surface tension of the dispersed particle suspension in the slurry is greater than that of the solvent, the slurry will flow to the edge, which eventually leads to the thick edge phenomenon.

[0005] In view of the above problems, the prior art mainly adopts the following solutions: first, the slit size is reduced to increase the outlet velocity of the slurry in the die, thereby reducing the drag force ratio of the slurry, but this will cause the internal pressure of the die to increase, which will easily cause the die outlet shape to expand, and a higher precision coating device is required; second, the coating gap is reduced, but the effect is limited; third, the outlet shape of the slit gasket is optimized to change the slurry flow state and weaken the edge expansion effect.

[0006] However, the above methods often improve only one aspect, lack a systematic solution, and do not fully consider the comprehensive influence of the rheological properties of the slurry, the pressure fluctuation in the coating process, and the temperature gradient in the drying process on the thick edge phenomenon. In addition, the optimization of the gasket shape in the prior art mainly focuses on the simple adjustment of the outlet geometric size, lacks precise control of the internal flow field, and is difficult to fundamentally solve the problem of excessive edge thickness.

[0007] ​Therefore, it is urgent to develop a systematic method that comprehensively considers the characteristics of slurry, die design, coating dynamic control and drying conditions, and effectively prevents the positive plate edge from being too thick, improves the quality and consistency of the positive plate coating. SUMMARY

[0008] The present application aims to: in view of the deficiencies of the prior art, provide a multi-dimensional collaborative prevention and control processing method from slurry pretreatment, rheological control, die optimization, pressure control to differentiated drying, realize the precise control of the uniformity of the positive plate thickness, and improve the battery performance and safety.

[0009] To achieve the above object, the present application provides the following technical scheme:

[0010] A processing method for preventing the positive plate edge from being too thick, comprising the following steps:

[0011] S1, slurry pretreatment: filtering the positive slurry through a precision filter screen with a pore size of 3-8 pm, and adding 0.5wt%-1.5wt% of a non-ionic surfactant, and stirring uniformly at 25-30°C;

[0012] S2, slurry rheological control: shearing the slurry to a shear viscosity of 1200-1800 mPa·s, and controlling the surface tension to 28-35 mN / m;

[0013] S3, die gasket optimization: using an inner concave wedge-shaped gasket, the gasket edge thickness is 10-30% thinner than the central area, forming a fluid resistance gradient field;

[0014] S4, pressure pulsation control coating: using a pressure feedback system to monitor the pressure fluctuation in the die in real time, controlling the pressure pulsation amplitude to be not more than ±2%, and dynamically adjusting the extrusion speed according to the monitoring results, with a response time of not more than 100 ms;

[0015] S5, differentiated drying treatment: passing the coated positive plate through a multi-section drying system, the edge area drying temperature is 15-25°C lower than the central area, forming a drying gradient field from the center to the edge.

[0016] Preferably, the non-ionic surfactant added in S1 is one or more of polyoxyethylene ether, polyethylene glycol or polypropylene oxide, with an average molecular weight of 1000-5000 and a critical micelle concentration of 0.05-0.2wt%.

[0017] Preferably, in S2, the slurry is sheared by a high-shear disperser, with a shear rate of 1000-3000 s - , and a shear time of 10-30 minutes.

[0018] Preferably, the regulation of the surface tension in S2 is achieved by adding 0.2wt%-0.8wt% of an organic silicon surface modifier, which is polysiloxane or modified polysiloxane, and the particle size is 100-500nm.

[0019] Preferably, the inner concave wedge-shaped gasket in S3 is processed by a laser cutting process, the gasket thickness gradually decreases from the center to the edge, and the thickness ratio of the edge to the center is 0.7-0.9, and the gasket material is wear-resistant tungsten steel alloy, and the surface roughness Ra is ≤0.2μm.

[0020] Preferably, the pressure feedback system in S4 includes a pressure sensor and a microfluidic control unit, the sensor sensitivity is ±0.1%, the sampling frequency is 100Hz, and the signal transmission delay is less than 5ms.

[0021] Preferably, the multi-section drying system in S5 is divided into an edge area and a central area, the surface temperature distribution of the positive plate is monitored in real time by an infrared thermal imager, the edge area uses a cold air auxiliary temperature control device, a temperature gradient field of 110-130℃ in the central area and 85-105℃ in the edge area is formed, and the drying time is 3-5 minutes.

[0022] Preferably, the S4 also includes dynamic coating gap regulation, a displacement sensor is used to monitor the flatness of the current collector surface, the gap control range is 50-200μm, the control accuracy is ±2μm, and the response time is less than 50ms.

[0023] Preferably, a step S2a of slurry rheological property testing is added between S2 and S3, a rotary rheometer is used to measure the thixotropy index of the slurry to be between 0.3-0.6, the thixotropic recovery rate is greater than 85%, and the structure recovery time after standing is controlled within 10-30 seconds.

[0024] Preferably, the positive electrode slurry in S1 is composed of 85-92wt% of positive electrode active material, 3-6wt% of conductive agent, and 4-8wt% of binder, and the solvent is NMP, and the solid content is 50-60wt%.

[0025] Preferably, S5 is followed by step S6 of positive plate edge thickness evaluation and quality control, a laser triangulation system is used to detect the thickness distribution of the positive plate, the difference rate of the edge thickness and the central thickness is controlled within ±3%, and the thickness uniformity score is not less than 90 points.

[0026] In addition, the application also provides a positive plate prepared by the processing method for preventing the edge of the positive plate from being too thick.

[0027] Compared with the prior art, the application has at least the following beneficial effects:

[0028] 1) The present application optimizes the rheological properties and interfacial properties of the slurry from the source by precise filtration and nonionic surfactant addition, combined with high shear pretreatment and organic silicon surface modifier to regulate surface tension, so that the slurry has more stable flow performance and more uniform spreading behavior in the coating process, effectively inhibiting the edge swelling effect caused by uneven slurry properties, reducing the deviation of edge thickness. Specifically, precise filtration removes large particles and agglomerates in the slurry, nonionic surfactant improves the wettability and flowability of the slurry, high shear pretreatment makes the particle dispersion more uniform, and organic silicon surface modifier precisely regulates the surface tension of the slurry. The comprehensive effect of these measures makes the slurry have ideal rheological properties and interfacial properties, reducing the probability of thick edge phenomenon from the source.

[0029] 2) The present application innovatively uses an inner concave wedge-shaped gasket, which gradually reduces the thickness of the gasket from the center to the edge, forming a specific fluid resistance gradient field. By controlling the change of the flow area, the flow rate at different positions is accurately adjusted. The flow rate in the center area is large, and the flow rate in the edge area is small, which fundamentally solves the edge swelling problem caused by traditional parallel gaskets. This design optimizes the flow field inside the die, significantly reduces the stress accumulation of the slurry in the edge area, and effectively inhibits the edge over-thickness phenomenon. According to the principle of fluid mechanics, at the outlet of the die, the traditional parallel gasket will cause the slurry at the edge to be subjected to additional wall shear stress, resulting in greater swelling effect; while the inner concave wedge-shaped gasket reduces the flow passage height in the edge area, increases the local fluid resistance, and reduces the flow rate in the edge area, making the slurry flow at the outlet more uniform, effectively inhibiting the thick edge phenomenon caused by edge swelling effect.

[0030] 3) The present application introduces an advanced pressure pulsation control system, which monitors the pressure fluctuation in the die in real time through a pressure sensor and dynamically adjusts the extrusion speed, overcoming the thickness unevenness problem caused by pressure fluctuation in the traditional constant speed extrusion method. At the same time, combined with dynamic coating gap control, it adapts to the slight undulations of the collector surface, realizing accurate control of the coating process and further improving the uniformity of the coating. The pressure pulsation control system can control the pressure fluctuation in the die within ±2%, which is significantly better than the traditional process, greatly reducing the thickness deviation caused by pressure fluctuation. The dynamic coating gap control system can adjust the position of the coating head in real time to compensate for the slight undulations of the collector surface, maintaining constant coating conditions. The combination of these two technologies forms a double dynamic control mechanism, making the coating process more stable and controllable, and further reducing the occurrence of thick edge phenomenon.

[0031] 4) The application designs an innovative differential drying method. In order to solve the problem of slurry migration caused by too fast solvent evaporation in the edge area during drying, a multi-zone drying system is used to form a temperature gradient field from the center to the edge, effectively slowing down the drying rate of the edge area and inhibiting the tendency of slurry flowing to the edge, thereby further controlling the edge thickness during the drying stage. In the traditional single-temperature drying process, the edge area is heated from three sides, and the solvent evaporates faster, which easily leads to the migration of slurry to the edge to form a thick edge; while the differential drying system of the application reduces the temperature of the edge area by 15~25℃, significantly slows down the drying rate of the edge area, and makes the entire positive plate dry more evenly, effectively inhibiting the tendency of slurry flowing to the edge, and further controlling the thick edge phenomenon from the drying link.

[0032] 5) The application realizes precise control of the edge thickness of the positive plate through the multi-dimensional synergy of slurry property optimization, die structure improvement, coating dynamic control and drying condition adjustment, the thickness difference rate between the edge and the center is controlled within ±3%, which is significantly better than the ±10% level of the prior art, and the uniformity of the electrode is improved, thereby improving the energy density, cycle performance and safety of the battery. This multi-dimensional synergy control method fully considers each link and influencing factor of the thick edge phenomenon, forming a systematic solution, not only solving the problem of limited effect of single measure in the prior art, but also realizing the optimized cooperation of each process link, producing a significant synergistic effect, and making the thickness uniformity of the positive plate reach a new level. BRIEF DESCRIPTION OF DRAWINGS

[0033] Figure 1 Structure diagram of the positive plate with over-thick edge. DETAILED DESCRIPTION

[0034] The technical solutions of the application will be described below in conjunction with specific embodiments. Obviously, the described embodiments are only a part of the embodiments of the application, not all embodiments. Based on the embodiments in the application, all other embodiments obtained by those skilled in the art without creative labor are within the scope of protection of the application.

[0035] In the first aspect according to the present application, a processing method for preventing the positive plate from being too thick at the edge is provided, comprising the following steps:

[0036] S1: slurry pretreatment, filtering the positive slurry through a precision filter screen with a pore size of 3~8μm, and adding 0.5wt%~1.5wt% of non-ionic surfactant, stirring uniformly at 25~30℃;

[0037] S2: rheological regulation of slurry, shear pretreatment of slurry to shear viscosity of 1200-1800 mPa·s, surface tension control in 28-35 mN / m;

[0038] S3: die gasket optimization, using concave wedge-shaped gasket, gasket edge thickness is 10-30% thinner than central area, forming fluid resistance gradient field;

[0039] S4: pressure pulsation control coating, using pressure feedback system to monitor pressure fluctuation in die in real time, controlling pressure pulsation amplitude not more than ±2%, and dynamically adjusting extrusion speed according to monitoring results, response time not more than 100 ms;

[0040] S5: differential drying treatment, after coating, the positive plate is passed through a multi-section drying system, the edge area drying temperature is 15-25℃ lower than the central area, forming a drying gradient field from the center to the edge.

[0041] In an embodiment according to the application, the non-ionic surfactant added in S1 is one or more of polyoxyethylene ether, polyethylene glycol or polypropylene oxide, with an average molecular weight of 1000-5000 and a critical micelle concentration of 0.05-0.2wt%. The non-ionic surfactant can reduce the surface tension of the slurry, improve the spreading property of the slurry on the substrate, while not affecting the electrochemical performance of the electrode material. Larger molecular weight and appropriate critical micelle concentration can ensure stable interfacial tension during drying process, preventing the slurry from flowing to the edge. In addition, the non-ionic surfactant does not contain ionized groups, has good electrochemical stability, and will not introduce additional electrochemical side reactions. Polyoxyethylene ether, polyethylene glycol and polypropylene oxide have good solubility in NMP solvent, high compatibility with active material and binder, and can form a uniform and stable slurry system. By precisely controlling the type, molecular weight and addition amount of surfactant, the surface tension of the slurry can be reduced to the most suitable range for coating, which can ensure good spreading property and inhibit excessive flow at the edge, thereby reducing the occurrence of thick edge phenomenon.

[0042] In an embodiment according to the application, the slurry is subjected to shear pretreatment by a high-shear disperser in S2, with a shear rate of 1000-3000 s -¹, the shearing time is 10-30 minutes. The high shearing pretreatment can break the loose agglomerates in the slurry, make the rheological property of the slurry more uniform and stable, reduce the stress difference in the flow process, and help to inhibit the edge swelling effect. Specifically, the high shearing disperser generates strong shearing force between the high-speed rotating rotor and stator to break and disperse the slurry. The shearing rate of 1000-3000 s-1 is the range optimized through experiments, and the dispersion effect is not obvious when it is lower than 1000 s-1, and the active material may be broken or the molecular chain of the binder may be broken when it is higher than 3000 s-1. The shearing time of 10-30 minutes can ensure that the slurry is sufficiently dispersed but not excessively damaged. This pretreatment process makes the particle distribution in the slurry more uniform, the rheological property more stable, and the flow behavior difference between the edge and center areas in the die flow process reduced, thereby reducing the edge swelling effect and effectively reducing the thick edge phenomenon.

[0043] In an embodiment according to the present application, the regulation of the surface tension in S2 is achieved by adding 0.2wt%-0.8wt% of an organic silicon surface modifier, which is polysiloxane or modified polysiloxane with a particle size of 100-500 nm. The organic silicon surface modifier can form a stable molecular film at the slurry-air interface, effectively reduce the surface tension, and will not affect the electrochemical performance of the slurry. The nano-scale particle size ensures that the modifier can be uniformly dispersed and will not affect the microstructure of the coating. The organic silicon surface modifier is a kind of material with special interfacial activity, which contains both hydrophilic groups and hydrophobic groups in its molecular structure, can be oriented on the surface of the slurry, and can significantly reduce the surface tension of the slurry. The polysiloxane backbone has good flexibility and thermal stability and is not easy to decompose during drying, and can continuously play a surface activity role. The particle size is controlled in the range of 100-500 nm, which can ensure good dispersibility and will not significantly affect the specific surface area and porosity of the electrode material. The addition amount of 0.2wt%-0.8wt% is the best range determined through experiments, and the effect is not obvious when it is lower than 0.2wt%, and the electrode structure may be loose when it is higher than 0.8wt%, which affects the battery performance. This surface tension regulation strategy cooperates with the non-ionic surfactant to significantly improve the rheological properties of the slurry and effectively inhibit the tendency of the slurry to migrate to the edge during drying.

[0044] In an embodiment according to the application, the inner concave wedge-shaped gasket in S3 is processed by a laser cutting process, the gasket thickness gradually decreases from the center to the edge, the thickness ratio of the edge to the center is 0.7-0.9, the gasket material is wear-resistant tungsten steel alloy, and the surface roughness Ra is ≤0.2 μm. The design of the inner concave wedge-shaped gasket can form a specific fluid resistance gradient field inside the die, the resistance in the center area is large, and the resistance in the edge area is small, so that the slurry flow rate at the edge is lower than that in the center area, effectively inhibiting the edge swelling effect. In specific implementation, the inner concave wedge-shaped gasket is processed by a laser cutting process, which can achieve micron-level processing precision and ensure smooth transition of the gasket thickness from the center to the edge. The thickness ratio of the gasket edge to the center is controlled in the range of 0.7-0.9, which is the best range verified by fluid mechanics calculation and experiment. Too small will result in too low slurry flow rate in the edge area, affecting the uniformity of coating, and too large will not have obvious inhibitory effect. The gasket material is wear-resistant tungsten steel alloy, which has high hardness, good wear resistance and is not easy to deform, and can maintain stable shape and size during long-term use. The surface roughness is controlled to be Ra≤0.2 μm, which ensures smooth flow of slurry on the surface of the gasket and avoids local flow instability caused by uneven surface roughness. This specially designed gasket fundamentally changes the flow field distribution inside the die and realizes source control of the edge swelling effect, which is one of the core technologies of the application.

[0045] In an embodiment according to the application, the pressure feedback system in S4 includes a pressure sensor and a microfluid control unit, the sensor sensitivity is ±0.1%, the sampling frequency is 100 Hz, and the signal transmission delay is less than 5 ms. High-precision pressure monitoring and fast-response control system can adjust the slurry extrusion speed in real time to inhibit the thickness unevenness caused by pressure fluctuation.

[0046] The specific composition and working principle of the pressure feedback system are as follows: the system mainly consists of a pressure sensor, a signal conditioning circuit, a microprocessor control unit and an actuator. The pressure sensor is installed in the slurry flow channel inside the die, and a high-precision piezoresistive or capacitive sensor is used, with a sensitivity of ±0.1% and a measurement range of 0-10 MPa, which can accurately capture the small pressure changes in the slurry flow process. The signal conditioning circuit amplifies, filters and digitizes the analog signal output by the sensor, and the digitized digital signal is transmitted to the microprocessor control unit. The microprocessor control unit uses an industrial-grade DSP or ARM processor and runs a specific PID control algorithm. By comparing the real-time pressure with the set pressure, the extrusion speed to be adjusted is calculated. The actuator includes a servo motor and a precision screw pump, which receives the instructions from the control unit and accurately adjusts the extrusion speed of the slurry.

[0047] The whole feedback control process is as follows: when the pressure fluctuation in the die is monitored (such as ±2% exceeding the set value), the system will immediately calculate the extrusion speed that needs to be adjusted, and implement the adjustment through the actuator. The sampling frequency of 100 Hz ensures that millisecond-level pressure fluctuations can be captured, the signal transmission delay is controlled within 5 ms, the whole system response time is not more than 100 ms, and the coating thickness unevenness caused by pressure fluctuation can be effectively inhibited. For example, when a 2% pressure rise is detected, the system will immediately reduce the extrusion speed by about 1.5-2%, so that the pressure quickly falls to the set range; when a 2% pressure drop is detected, the system will increase the extrusion speed by about 1.5-2%, so that the pressure rises to the set range. This precise pressure control ensures the stable flow of the slurry during coating, significantly reduces the thickness deviation caused by pressure fluctuation, and further suppresses the edge over-thickness phenomenon.

[0048] In an embodiment according to the present application, the multi-section drying system in S5 is divided into an edge zone and a central zone, the surface temperature distribution of the positive electrode sheet is monitored in real time by an infrared thermal imager, the edge zone adopts a cold air auxiliary temperature control device, a temperature gradient field of 110-130°C in the central zone and 85-105°C in the edge zone is formed, and the drying time is 3-5 minutes. The differential drying design can delay the solvent evaporation rate in the edge region, slow down the tendency of the slurry to migrate to the edge, and effectively control the edge thickness.

[0049] The specific structure and working principle of the multi-section drying system are as follows: the system is composed of a central heating zone, an edge temperature control zone, an infrared thermal image monitoring system and a cold air auxiliary temperature control device. The central heating zone adopts a precision temperature controlled infrared heater or a hot air nozzle array, the temperature is controlled in the range of 110-130°C, the precision is ±2°C, and it is mainly responsible for heating and drying the central region of the positive electrode sheet. The edge temperature control zone adopts an independently controlled low temperature heating element, the temperature is controlled in the range of 85-105°C, the precision is ±2°C, and the purpose is to perform low temperature drying treatment on the edge region. The infrared thermal image monitoring system is composed of a high resolution infrared thermal imager and an image processing system, the resolution is not less than 640×480 pixels, the temperature measurement precision is ±1°C, the scanning frequency is 10Hz, the temperature distribution of the positive electrode sheet surface is monitored in real time, and the data is fed back to the control system. The cold air auxiliary temperature control device is composed of a precision air volume controller, a temperature adjusting unit and a directional air nozzle, which can accurately send cold air with a temperature of 20-30°C to the edge region of the positive electrode sheet, the wind speed can be adjusted in the range of 0.5-2m / s, and is used to further control the drying rate of the edge region.

[0050] The system workflow is as follows: after the positive plate enters the drying system, the infrared thermal imager monitors the surface temperature distribution in real time, and the data is transmitted to the control system. The control system adjusts the heating power of the central and edge areas according to the temperature distribution, and controls the air volume and temperature of the cold air auxiliary device at the same time, to ensure the formation of a temperature gradient field with a central area temperature of 110-130°C and an edge area temperature of 85-105°C. This precisely controlled temperature gradient significantly reduces the solvent evaporation rate in the edge area, effectively inhibiting the phenomenon of slurry migration to the edge. Compared with traditional single-temperature drying, this differential drying method can reduce the edge thickness deviation by 20-40%, significantly improving the thickness uniformity of the positive plate.

[0051] In an embodiment according to the present application, dynamic coating gap control is further included in S4. A displacement sensor is used to monitor the flatness of the current collector surface, the gap control range is 50-200μm, the control accuracy is ±2μm, and the response time is less than 50ms. Dynamic gap control can adapt to the slight ups and downs of the current collector surface, maintain constant coating pressure, and further improve thickness uniformity.

[0052] The specific structure and working principle of the dynamic coating gap control system are as follows: the system is composed of a displacement sensor, a signal processing circuit, a controller and a precision driving mechanism. The displacement sensor uses the principle of laser triangulation, is installed in front of the coating head, and is used to detect the flatness of the current collector surface in real time, with a measurement accuracy of ±1μm and a measurement frequency of 200Hz. The signal processing circuit filters and processes the signals output by the sensor and converts them into standard digital signals. The controller calculates the position adjustment of the coating head based on the processed displacement data and sends control signals to the precision driving mechanism. The precision driving mechanism uses a high-response piezoelectric ceramic actuator or a precision electric push rod, which can complete micron-level position adjustment within 50ms.

[0053] The system operation process is as follows: when the displacement sensor detects that the current collector surface has ups and downs or is not flat, the control system immediately calculates the position adjustment of the coating head and implements the adjustment through the precision driving mechanism to keep the coating gap constant at the set value (usually 50-200μm, which can be adjusted according to process requirements). For example, when a 5μm upward protrusion is detected on the current collector surface, the system will move the coating head upward by 5μm within 30ms to keep the coating gap unchanged; when a 3μm downward indentation is detected on the current collector surface, the system will move the coating head downward by 3μm within 30ms. This dynamic gap control can effectively cope with the slight ups and downs of the current collector surface, maintain constant coating pressure and slurry flow conditions, further improve the uniformity of the coating thickness, and reduce the occurrence of edge over-thickness. This technology, combined with pressure pulsation control, forms a double dynamic control mechanism to ensure high precision and high stability of the coating process.

[0054] In an embodiment according to the present application, a step S2a of slurry rheological property test is added between S2 and S3: the thixotropy index of the slurry is measured by a rotary rheometer to be between 0.3 and 0.6, the thixotropic recovery rate is greater than 85%, and the structure recovery time after standing is controlled within 10-30 seconds. Precise control of the rheological properties of the slurry is the key to ensuring uniformity of coating, and appropriate thixotropy can ensure that the structure of the slurry recovers rapidly after the shear force disappears, preventing excessive leveling from causing increased edge thickness.

[0055] The specific method and significance of the slurry rheological property test are as follows: the test is performed by a rotary rheometer (such as Anton Paar MCR series or TA Instruments DHR series), and the test temperature is consistent with the coating temperature (usually 25°C). The thixotropy index is the ratio of the hysteresis loop area formed by the flow curve of the slurry during the rising and falling processes to the area under the rising curve, reflecting the structure damage and reconstruction ability of the slurry. The slurry with a thixotropy index between 0.3 and 0.6 has appropriate rheological properties, good fluidity when subjected to shear during coating, and rapid recovery of a certain structure after the shear force disappears, preventing excessive leveling. The thixotropic recovery rate is the ratio of the viscosity recovered within a certain time after standing to the original viscosity after the slurry has undergone high shear, which is required to be greater than 85% to ensure that the slurry can maintain its shape after coating. The structure recovery time is the time required for the slurry to return to the initial structure state from the high shear state, which is most appropriate within 10-30 seconds, too short will cause the slurry to solidify rapidly after coating, which is not conducive to eliminating surface defects, and too long will cause the slurry to level excessively, causing the thick edge phenomenon.

[0056] Through this test step, the slurry can be ensured to have appropriate rheological properties to meet the requirements of the coating process. If the test finds that the thixotropy index, thixotropic recovery rate, or structure recovery time of the slurry does not meet the requirements, adjustments can be made by adjusting the shear pretreatment parameters, changing the additive ratio, or adjusting the solid content, etc., until the requirements are met. The addition of this step makes the slurry preparation process more scientific and controllable, laying a good foundation for the subsequent coating process, and significantly improving the thickness uniformity of the positive electrode sheet.

[0057] In an embodiment according to the present application, the positive electrode slurry composition in S1 is: NCM811 or NCA active material 85-92wt%, conductive agent 3-6wt%, binder 4-8wt%, solvent NMP, and solid content 50-60wt%.

[0058] The positive electrode slurry composition has an important influence on the coating uniformity. The NCM811 or NCA active material has a higher density and specific capacity, and is the mainstream positive electrode material of the current high-energy-density lithium ion battery. The active material content of 85-92wt% is the optimized range. Less than 85wt% will result in insufficient battery energy density, and more than 92wt% will result in unstable electrode structure and decreased conductivity. The conductive agent content is 3-6wt%, which is mainly used to improve the electronic conductivity of the electrode. Less than 3wt% will affect the battery performance due to incomplete conductive network, and more than 6wt% will occupy too much space to reduce the energy density. The binder content is 4-8wt%, which is mainly used to bond the components and ensure the stability of the coating structure. Less than 4wt% will result in insufficient bonding strength, and more than 8wt% will hinder the transmission of lithium ions. NMP is selected as the solvent because it has good solubility to PVDF binder and moderate boiling point for easy drying control. The solid content is controlled at 50-60wt% considering the rheological properties and drying characteristics of the slurry. Less than 50wt% will result in too large drying shrinkage, and more than 60wt% will affect the coating uniformity due to insufficient slurry flowability.

[0059] In an embodiment according to the present application, the S5 is further followed by a step S6 of positive electrode sheet edge thickness evaluation and quality control. A laser triangulation system is used to detect the thickness distribution of the positive electrode sheet, the difference rate of the edge thickness and the central thickness is controlled within ±3%, and the thickness uniformity score is not less than 90 points. The accurate thickness evaluation system can objectively quantify the coating uniformity and provide a basis for process adjustment.

[0060] Step S6 is a key link for quality evaluation and control of the processed positive electrode sheet, and the specific implementation method is as follows: a laser triangulation system is used to detect the thickness distribution of the positive electrode sheet, which is composed of a high-precision laser displacement sensor, a precision moving platform and data acquisition and analysis software. During measurement, the positive electrode sheet is fixed on the platform, and the laser sensor measures a point every 5mm along the width direction of the positive electrode sheet. Each positive electrode sheet is measured at least 10 lines to obtain complete thickness distribution data. The average thickness of the edge region (within 5mm from the edge) and the central region is calculated by the special analysis software, and the difference rate of the two is calculated.

[0061] The thickness difference rate calculation formula is: difference rate = [(edge average thickness-central average thickness) / central average thickness] x 100%

[0062] The thickness uniformity score is based on a 100-point system, and the calculation method is: score = 100-5 x thickness difference rate (%)

[0063] When the thickness difference rate is 0, the score is 100 points; when the difference rate is 20%, the score is 0 point. The thickness difference rate is required to be controlled within ±3%, and the uniformity score is not less than 90 points. This strict quality control standard ensures the high uniformity of the positive plate, and provides guarantee for the performance and safety of the battery.

[0064] In addition, the S6 step also includes mechanical property tests such as 180° bending test and peeling strength test on the positive plate to evaluate the structural stability and bonding strength of the edge area. These test results serve as an important basis for process adjustment, and through closed-loop control, the process parameters are continuously optimized to achieve continuous improvement of the quality of the positive plate.

[0065] In a second aspect according to the present application, the present application also provides a positive plate prepared by the processing method for preventing over-thickness of the edge of the positive plate described in any of the above paragraphs.

[0066] The implementation and advantages of the present application will be further described below in conjunction with specific examples.

[0067] Example 1

[0068] S1: slurry pretreatment, the NCM811 positive slurry (NCM811 88wt%, conductive carbon black 5wt%, PVDF binder 7wt%, solvent NMP, solid content 55wt%) is filtered twice through a precision filter screen with a pore size of 5μm, 1.0wt% of polyoxyethylene ether (molecular weight 2000, critical micelle concentration 0.1wt%) is added, and stirred at 28℃ for 30 minutes to uniformity;

[0069] S2: rheological regulation of slurry, using a high-shear disperser to shear the slurry, the shear rate is 2000s - ¹, the shear time is 20 minutes, the shear viscosity is 1500mPa·s; 0.5wt% of polysiloxane surface modifier (particle size 300nm) is added to regulate the surface tension to 32mN / m;

[0070] S2a: rheological property test of slurry, using Anton Paar MCR302 rotary rheometer to measure the thixotropic index of the slurry as 0.45, the thixotropic recovery rate is 90%, and the structure recovery time after standing is 20 seconds;

[0071] S3: die pad optimization, using a concave wedge-shaped pad, the pad material is tungsten steel alloy, the surface roughness Ra=0.15μm, the center thickness of the pad is 200μm, the edge thickness is 160μm, and the thickness ratio of the edge to the center is 0.8;

[0072] S4: Pressure pulsation control coating, real-time monitoring of pressure fluctuation in the die using a pressure feedback system, sensor sensitivity of ±0.1%, sampling frequency of 100Hz, control pressure pulsation amplitude of ±1.5%; at the same time, using displacement sensor to monitor the flatness of aluminum foil surface, dynamically adjusting the coating gap within 120±2μm, response time of 40ms;

[0073] S5: Differential drying treatment, using multi-zone drying system, the temperature of central zone is set to 120℃, the temperature of edge zone is set to 100℃, real-time monitoring of temperature distribution by infrared thermal imager, drying time of 4 minutes;

[0074] S6: Positive electrode sheet thickness evaluation, using laser triangulation measurement system to detect the thickness distribution of positive electrode sheet, the edge thickness is 65.2μm, the central thickness is 64.5μm, the thickness difference rate is 1.1%, the thickness uniformity score is 95.

[0075] Example 2

[0076] S1: Slurry pretreatment, NCA positive electrode slurry (NCA 90wt%, acetylene black 4wt%, PVDF binder 6wt%, solvent NMP, solid content 58wt%) is filtered three times through a precision filter screen with a pore size of 4μm, 0.8wt% polyethylene glycol (molecular weight 3000, critical micelle concentration 0.08wt%) is added, and stirred at 26℃ for 35 minutes to uniformity;

[0077] S2: Rheological regulation of slurry, using high shear disperser for shear pretreatment of slurry, shear rate of 1800s - ¹, shear time of 18 minutes, shear viscosity of 1650mPa·s; adding 0.4wt% modified polysiloxane surface modifier (particle size 250nm), the surface tension is regulated to 30mN / m;

[0078] S2a: Rheological property test of slurry, using rotary rheometer to measure the thixotropy index of slurry is 0.4, the thixotropic recovery rate is 92%, the structure recovery time after standing is 18 seconds;

[0079] S3: Die pad optimization, using concave wedge-shaped pad, pad material is tungsten steel alloy, surface roughness Ra=0.12μm, pad center thickness is 180μm, edge thickness is 135μm, thickness ratio of edge to center is 0.75;

[0080] S4: Pressure pulsation control coating, real-time monitoring of pressure fluctuation in the die using a pressure feedback system, sensor sensitivity of ±0.1%, sampling frequency of 100Hz, control pressure pulsation amplitude of ±1.2%; at the same time, using displacement sensor to monitor the flatness of aluminum foil surface, dynamically adjusting the coating gap within 100±2μm range, response time is 35ms;

[0081] S5: Differential drying treatment, using multi-zone drying system, the temperature of central zone is set to 125℃, the temperature of edge zone is set to 105℃, real-time monitoring of temperature distribution by infrared thermal imager, drying time is 3.5 minutes;

[0082] S6: Positive electrode sheet thickness evaluation, using laser triangulation measurement system to detect the thickness distribution of positive electrode sheet, the edge thickness is 68.1μm, the central thickness is 67.5μm, the thickness difference rate is 0.9%, the thickness uniformity score is 97 points.

[0083] Example 3

[0084] S1: Slurry pretreatment, NCM811 positive electrode slurry (NCM811 86wt%, graphene conductive agent 3wt%, carbon nanotube 2wt%, PVDF binder 9wt%, solvent is NMP, solid content 52wt%) is filtered twice through a precision filter screen with a pore size of 3μm, 1.2wt% of polyoxypropylene (molecular weight 4000, critical micelle concentration 0.15wt%) is added, and stirred at 30℃ for 25 minutes until uniform;

[0085] S2: Rheological regulation of slurry, using a high-shear disperser to shear the slurry, shear rate of 2500s⁻¹, shear time of 25 minutes, shear viscosity of 1350mPa·s; adding 0.6wt% of polysiloxane surface modifier (particle size 200nm), the surface tension is regulated to 28mN / m;

[0086] S2a: Rheological property test of slurry, using a rotary rheometer to measure the thixotropy index of the slurry as 0.35, the thixotropic recovery rate is 95%, the structure recovery time after standing is 15 seconds;

[0087] S3: Die pad optimization, using an inner concave wedge-shaped pad, the pad material is tungsten steel alloy, the surface roughness Ra=0.1μm, the pad center thickness is 220μm, the edge thickness is 176μm, the thickness ratio of edge to center is 0.8;

[0088] S4: Pressure pulsation control coating, real-time monitoring of pressure fluctuation in the die by using pressure feedback system, sensor sensitivity of ±0.1%, sampling frequency of 100 Hz, control pressure pulsation amplitude of ±1.0%; at the same time, using displacement sensor to monitor the flatness of aluminum foil surface, dynamically adjusting the coating gap within 150±2μm range, response time of 30ms;

[0089] S5: Differential drying treatment, using multi-zone drying system, the temperature of the central zone is set to 130℃, the temperature of the edge zone is set to 105℃, real-time monitoring of temperature distribution by using infrared thermal imager, drying time of 3.8 minutes;

[0090] S6: Positive electrode sheet thickness evaluation, using laser triangulation measurement system to detect the thickness distribution of the positive electrode sheet, the edge thickness is 74.2μm, the central thickness is 73.8μm, the thickness difference rate is 0.5%, the thickness uniformity score is 98.

[0091] Comparative Example 1

[0092] Using conventional parallel gasket coating process, as follows:

[0093] S1: The NCM811 positive electrode slurry (NCM811 88wt%, conductive carbon black 5wt%, PVDF binder 7wt%, solvent NMP, solid content 55wt%) is filtered through a filter screen with a pore size of 20μm, without adding a surfactant, and stirred uniformly at room temperature;

[0094] S2: No special rheological control, the shear viscosity of the slurry is 2200mPa·s, and the surface tension is 45mN / m;

[0095] S3: Using traditional parallel gasket, gasket thickness uniformity of 200μm;

[0096] S4: Using constant speed extrusion, without pressure feedback control, coating gap fixed at 120μm;

[0097] S5: Using single temperature zone drying, drying temperature uniformity of 120℃, drying time of 4 minutes;

[0098] S6: Positive electrode sheet thickness evaluation, the edge thickness is 78.5μm, the central thickness is 65.2μm, the thickness difference rate is 20.4%, the thickness uniformity score is 65.

[0099] Comparative Example 2

[0100] Only using the method of surfactant to control the surface tension of the slurry, as follows:

[0101] S1: Filter the NCM811 cathode slurry (NCM811 88wt%, conductive carbon black 5wt%, PVDF binder 7wt%, solvent NMP, solid content 55wt%) through a 10μm pore size filter, add 1.0wt% of polyoxyethylene ether, and stir at room temperature until uniform;

[0102] S2: Without shear pretreatment, the slurry shear viscosity was measured to be 2000 mPa·s and the surface tension was 35 mN / m;

[0103] S3: Using traditional parallel gaskets with a uniform gasket thickness of 200 μm;

[0104] S4: constant speed extrusion method is adopted, without pressure feedback control, and the coating gap is fixed at 120μm;

[0105] S5: Single temperature zone drying is adopted, the drying temperature is uniformly 120℃, and the drying time is 4 minutes;

[0106] S6: Evaluation of the thickness of the positive electrode sheet: the edge thickness was measured to be 72.3 μm, the center thickness was 65.8 μm, the thickness difference rate was 9.9%, and the thickness uniformity score was 78 points.

[0107] Comparative Example 3

[0108] Only the concave gasket optimization method is used, as follows:

[0109] S1: NCM811 cathode slurry (NCM811 88wt%, conductive carbon black 5wt%, PVDF binder 7wt%, solvent NMP, solid content 55wt%) was filtered through a 10μm pore size filter without adding surfactant and stirred at room temperature until homogeneous;

[0110] S2: Without special rheological control, the measured slurry shear viscosity is 2200 mPa·s and the surface tension is 45 mN / m;

[0111] S3: uses a concave wedge-shaped gasket with a center thickness of 200 μm and an edge thickness of 160 μm;

[0112] S4: constant speed extrusion method is adopted, without pressure feedback control, and the coating gap is fixed at 120μm;

[0113] S5: Single temperature zone drying is adopted, the drying temperature is uniformly 120℃, and the drying time is 4 minutes;

[0114] S6: Evaluation of the thickness of the positive electrode sheet: the edge thickness was measured to be 70.1 μm, the center thickness was 66.0 μm, the thickness difference rate was 6.2%, and the thickness uniformity score was 84 points.

[0115] Comparative Example 4

[0116] Only the method of differential drying is used, as follows:

[0117] S1: The NCM811 positive electrode slurry (NCM811 88wt%, conductive carbon black 5wt%, PVDF binder 7wt%, solvent NMP, solid content 55wt%) is filtered through a filter screen with a pore size of 10pm, without adding a surfactant, and stirred uniformly at room temperature;

[0118] S2: No special rheological control is performed, and the slurry shear viscosity is measured to be 2200mPa·s, and the surface tension is 45mN / m;

[0119] S3: A traditional parallel gasket is used, with a uniform gasket thickness of 200pm;

[0120] S4: A constant speed extrusion method is used, without pressure feedback control, and the coating gap is fixed at 120pm;

[0121] S5: A multi-zone drying system is used, with a central zone temperature set to 120°C and an edge zone temperature set to 100°C, and a drying time of 4 minutes;

[0122] S6: Positive electrode sheet thickness evaluation, with an edge thickness of 74.5pm, a central thickness of 66.2pm, a thickness difference rate of 12.5%, and a thickness uniformity score of 75 points.

[0123] The positive electrode sheets prepared in the above examples and comparative examples are respectively cut into a specification of 3.5cm x 5.0cm, and assembled into soft pack batteries with suitable negative electrode sheets and separators, and the electrolyte is a 1.0mol / L LiPF6 solution of EC / DMC / EMC (volume ratio 1:1:1), which is packaged in an argon glove box.

[0124] The positive electrode sheets and batteries prepared in the above examples and comparative examples are subjected to the following performance tests, and the test results are shown in Tables 1 and 2.

[0125] 1. Positive electrode sheet thickness distribution measurement method: A laser triangulation measurement system is used to measure a point every 5mm along the width direction of the positive electrode sheet, and 10 lines are measured for each sample, and the average thickness and difference rate of the edge (region less than 5mm from the edge) and the central region are calculated.

[0126] 2. Thickness uniformity score calculation method: Based on the thickness difference rate calculation, score = 100-5 x thickness difference rate (%), the smaller the difference rate, the higher the score, and the full score is 100 points.

[0127] 3. Edge stress evaluation method: adopt the positive plate edge bending resistance test, bend the positive plate edge 180°, observe the active material shedding situation, and divide it into 5 grades according to the shedding degree (1-5 points, the higher the score, the better the bending resistance).

[0128] 4. First charge-discharge efficiency test method: use new battery test system, at 25℃, with 0.1C, first charge-discharge, calculate the first coulomb efficiency.

[0129] 5. Rate performance test method: discharge test at 0.5C, 1C, 2C, 5C and 10C rate, calculate the capacity retention rate (relative to 0.5C capacity).

[0130] 6. Cycle life test method: at 25℃, with 1C rate, charge-discharge 500 times, measure the capacity change every 50 times, calculate the capacity retention rate (500th cycle capacity / first cycle capacity).

[0131] 7. Battery safety evaluation method: use needle test (10mm / s speed, room temperature, 100% SOC), observe whether the battery ignites or explodes, and record the highest temperature. Safety score standard: no fire and explosion is qualified, the lower the highest temperature, the higher the score (1-5 points).

[0132] Table 1: Positive plate thickness uniformity and performance test results

[0133]

[0134] Table 2: Battery performance test results

[0135]

[0136] Among them, the above test results are analyzed as follows:

[0137] 1. Comparative analysis of examples and comparative examples:

[0138] (1) Thickness uniformity: the thickness difference rates of examples 1-3 are 1.1%, 0.9% and 0.5% respectively, all within the target range of ±3%, which is significantly better than comparative examples 1-4. This shows that the multi-dimensional synergistic prevention and control thick edge method of the application can effectively solve the problem of over-thick positive plate edge in traditional process. Especially compared with the single use of some optimization method in comparative example 2-4, the comprehensive solution of the application has more significant effect.

[0139] (2) Edge stress evaluation: the edge stress scores of examples 1-3 are 4.8, 4.9 and 5.0 respectively, which are significantly higher than those of comparative examples 1-4. This shows that the positive plate with uniform thickness has smaller active material shedding degree when bending, and has better mechanical stability, which is beneficial to the safety and long-term reliability of the battery.

[0140] (3) Battery performance: The batteries prepared in Examples 1-3 are significantly superior to Comparative Examples 1-4 in terms of first charge-discharge efficiency, rate capability, and cycle life. In particular, the capacity retention rate of the examples is significantly higher than that of the comparative examples after 10C high-rate discharge and 500 cycles, indicating that the positive electrode sheet with uniform thickness can significantly improve the lithium ion transmission efficiency and electrode stability, and improve the battery performance.

[0141] (4) Safety performance: Examples 1-3 exhibit better safety performance in the needle test, with scores of 4.7, 4.9, and 5.0, respectively, which is significantly better than Comparative Examples 1-4. This indicates that the positive electrode sheet with uniform thickness has lower risk of thermal runaway under abuse conditions such as needle puncture, and better safety performance.

[0142] 2. Comparative analysis between examples:

[0143] (1) Example 3 performs best in all performance indicators, with a thickness difference rate of only 0.5%, and the best battery performance and safety. This is mainly because Example 3 uses more precise process parameter control, including a smaller pore size (3 pm) filter, a higher shear rate (2500 s - ¹), a lower surface tension (28 mN / m), and more precise pressure control (±1.0%).

[0144] (2) Example 2 is second, and its excellent performance is mainly due to the use of NCA positive electrode material and more optimized gasket design (edge-to-center thickness ratio of 0.75), making the slurry flow control more precise.

[0145] (3) Example 1, although slightly inferior in performance among the three examples, still far exceeds all comparative examples in various indicators, indicating that the method of the present application has good universality and stability.

[0146] 3. Process parameter influence analysis:

[0147] (1) Effect of slurry pretreatment on thickness uniformity: From the comparison of Comparative Example 1 and Comparative Example 2, it can be seen that the addition of a surfactant alone can reduce the thickness difference rate from 20.4% to 9.9%, indicating that the interface properties of the slurry have a significant impact on the thick edge phenomenon. Further combined with precise filtration and rheological control (Examples 1-3), the difference rate can be reduced to below 1%.

[0148] (2) Effect of gasket design: Comparative Example 3 uses a concave gasket to reduce the thickness difference rate to 6.2%, which is significantly improved compared to 20.4% in Comparative Example 1, demonstrating that optimization of the die flow field is very effective in suppressing the edge swelling effect.

[0149] (3) The influence of drying process: The thickness difference rate of Comparative Example 4 was reduced to 12.5% by differential drying, which indicated that controlling the drying rate of the edge area could partially inhibit the slurry migration to the edge.

[0150] (4) Synergistic effect: The thickness difference rate of Comparative Examples 2-4 was only 6.2% (Comparative Example 3) by using single technical measures, while the thickness difference rate of Examples 1-3 was reduced to below 1% by combining all the optimization measures, which indicated that there was a significant synergistic effect in multi-dimensional synergistic control.

[0151] 4. Analysis of consistency between experimental data and theoretical expectations:

[0152] (1) The relationship between the rheological properties of the slurry and the thickness uniformity: Example 3 had the lowest thixotropic index (0.35) and the highest thixotropic recovery rate (95%), and also achieved the best thickness uniformity (0.5% difference rate). This is consistent with the theoretical expectation that lower thixotropy and faster structure recovery are beneficial to the rapid stabilization of the slurry after coating, reducing the thickness change during the leveling process.

[0153] (2) The relationship between the pressure control accuracy and the thickness uniformity: The pressure fluctuation of Examples 1-3 was controlled within ±1.0%-±1.5%, and the corresponding thickness uniformity was also good, which confirmed the importance of accurate pressure control for thickness uniformity.

[0154] (3) The relationship between the temperature gradient and the edge thickness: The temperature difference between the edge area and the central area in Examples 1-3 was 20-25°C, which effectively slowed down the solvent evaporation rate in the edge area and inhibited the tendency of the slurry to flow to the edge, thereby further improving the thickness uniformity.

[0155] 5. Analysis of performance improvement mechanism:

[0156] (1) The reason for the improvement of the first charge-discharge efficiency: The uniform thickness of the positive electrode sheet allows the electrolyte to be more evenly soaked, the utilization rate of the electrode active material is higher, and the interface reaction is more consistent, thereby improving the first charge-discharge efficiency. The first charge-discharge efficiency of Example 3 reached 93.1%, which was significantly higher than that of Comparative Example 1 (85.3%).

[0157] (2) The reason for the improvement of high-rate performance: Uniform electrode thickness means uniform lithium ion diffusion path and electron transmission distance, which reduces local polarization and concentration polarization, thereby improving the capacity retention rate at high rate. The capacity retention rate of Example 3 at 10C rate was 86.0%, which was much higher than that of Comparative Example 1 (65.7%).

[0158] (3) The reason for prolonging the cycle life: the uniform electrode structure reduces the uneven stress in the electrode, reduces the structural deformation and damage during charging and discharging, and avoids local overcharging and overdischarging, thereby prolonging the cycle life of the battery. The capacity retention rate of Example 3 is still 92.5% after 500 cycles, while that of Comparative Example 1 is only 72.1%.

[0159] (4) The reason for improving the safety performance: the positive plate with uniform edge thickness avoids local stress concentration during winding or lamination, reduces the risk of active material falling off and electrode deformation, and at the same time, the heat distribution is more uniform under abuse conditions, reducing the risk of thermal runaway, thereby improving the safety performance of the battery.

[0160] In summary, the processing method for preventing the positive plate edge from being too thick proposed by the present application successfully controls the thickness difference rate of the positive plate edge and the center within ±3% through the multi-dimensional synergistic effect of slurry pretreatment, rheological control, die pad optimization, pressure pulsation control and differential drying, which is significantly better than the prior art level. This highly uniform positive plate not only improves the energy density and rate performance of the battery, but also improves the cycle life and safety performance, and has important practical application value.

[0161] 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.

[0162] 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 above specific embodiments, 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 explanation and do not constitute any limitation on the present application.

Claims

1. A processing method for preventing the edge of a positive electrode sheet from being too thick, characterized in that: The following steps are involved: S1. Slurry pretreatment: Filter the positive electrode slurry through a precision filter with a pore size of 3-8 μm, add 0.5wt%-1.5wt% of a non-ionic surfactant, and stir evenly at 25-30°C. S2. Slurry rheology control: The slurry is shear pretreated to a shear viscosity of 1200-1800 mPa·s and a surface tension of 28-35 mN / m; S3. Die head gasket optimization: Use a concave wedge-shaped gasket, with the gasket edge thickness 10-30% thinner than the central area to form a fluid resistance gradient field; S4, pressure pulsation controlled coating: A pressure feedback system is used to monitor the pressure fluctuation in the die head in real time, controlling the pressure pulsation amplitude to no more than ±2%, and dynamically adjusting the extrusion speed based on the monitoring results, with a response time of no more than 100ms; S5. Differentiated drying treatment: The coated positive electrode sheet is passed through a multi-segment drying system. The drying temperature in the edge area is 15~25℃ lower than that in the central area, forming a drying gradient field from the center to the edge.

2. The method for preventing the edge of the positive electrode sheet from being too thick according to claim 1, characterized in that: The nonionic surfactant added in S1 is one or more of polyoxyethylene ether, polyethylene glycol or polyoxypropylene, with an average molecular weight of 1000-5000 and a critical micelle concentration of 0.05-0.2 wt %.

3. The method for preventing the edge of the positive electrode sheet from being too thick according to claim 1, characterized in that: In S2, the slurry is sheared and pretreated by a high shear disperser at a shear rate of 1000-3000s. - ¹, shearing time is 10~30 minutes.

4. The method for preventing the edge of a positive electrode sheet from being too thick according to claim 1, characterized in that: The surface tension of S2 is regulated by adding 0.2 wt% to 0.8 wt% of an organosilicon surface modifier, wherein the organosilicon surface modifier is polysiloxane or modified polysiloxane, and the particle size thereof is 100 to 500 nm.

5. The method for preventing the edge of a positive electrode sheet from being too thick according to claim 1, characterized in that: The concave wedge-shaped gasket in S3 is processed by laser cutting technology. The thickness of the gasket gradually decreases from the center to the edge, and the thickness ratio of the edge to the center is 0.7~0.

9. The gasket material is wear-resistant tungsten steel alloy, and the surface roughness Ra≤0.2μm.

6. The method for preventing the edge of a positive electrode sheet from being too thick according to claim 1, characterized in that: The pressure feedback system in S4 includes a pressure sensor and a microfluidic control unit. The sensor sensitivity is ±0.1%, the sampling frequency is 100 Hz, and the signal transmission delay is less than 5 ms.

7. The method for preventing the edge of a positive electrode sheet from being too thick according to claim 1, characterized in that: The multi-section drying system in the S5 is divided into an edge zone and a central zone. The surface temperature distribution of the positive electrode is monitored in real time by an infrared thermal imager. A cold air-assisted temperature control device is used in the edge zone to form a temperature gradient field with a central zone temperature of 110-130°C and an edge zone temperature of 85-105°C. The drying time is 3-5 minutes.

8. The method for preventing the edge of a positive electrode sheet from being too thick according to claim 1, characterized in that: The S4 also includes dynamic coating gap control, using a displacement sensor to monitor the surface flatness of the current collector, with a gap control range of 50-200 μm, a control accuracy of ±2 μm, and a response time of less than 50 ms.

9. The method for preventing the edge of a positive electrode sheet from being too thick according to claim 1, characterized in that: Add step S2a between S2 and S3: slurry rheological property test, using a rotational rheometer to measure the thixotropy index of the slurry to be between 0.3 and 0.6, the thixotropy recovery rate to be greater than 85%, and the structural recovery time after standing to be controlled within 10 to 30 seconds.

10. A positive electrode sheet, characterized in that: The positive electrode sheet is manufactured by the processing method for preventing the edge of the positive electrode sheet from being too thick according to any one of claims 1 to 9.

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