Battery positive electrode sheet safety layer slurry coating method and battery positive electrode sheet

Abstract

The present invention provides a method for coating a safety layer slurry on a battery positive electrode sheet and a battery positive electrode sheet. The method for coating a safety layer slurry on a battery positive electrode sheet includes preparing a safety layer slurry, extrusion coating, and baking. The safety layer slurry preparation comprises uniformly mixing a safety solute and a binder, gradually adding a solvent in two or more portions and stirring at high and low speeds to achieve a solution with a solid content of 3-8% and a viscosity of 50-200 cP. The extrusion coating utilizes a multi-layer extrusion die, wherein an active material layer slurry is coated between a foil to be coated and the safety layer slurry, the distance between the coating die and the active material layer is 50-130 μm, and the thickness of a gasket at the safety layer slurry outlet is 0.1-0.3 mm. The present invention improves the solid content and viscosity of the safety layer slurry, the coating die gap, and the thickness of the gasket, so that the safety layer coated by this method can have a thin thickness and good safety without affecting the energy density of the electrode sheet.

Description

Technical Field

[0001] The present invention relates to the field of electrochemistry, and in particular to a method for coating a safety layer slurry on a battery positive electrode sheet. The present invention also relates to a battery positive electrode sheet prepared by the method. Background Art

[0002] The number of lithium-ion power batteries installed in vehicles is increasing year by year, and the market share of power battery vehicles is also growing. With an increasing number of companies entering the power battery manufacturing market, competition is intensifying and the market entry threshold is rising. Currently, to increase the energy density of power batteries, most companies are using a high-nickel + silicon-carbon system to manufacture lithium-ion power batteries. However, high energy density also carries with it a high risk. Current measures to improve battery safety include the use of flame-retardant electrolytes, separator safety coatings, and electrode surface safety coatings.

[0003] Flame-retardant electrolyte technology has made slow progress in the industry, mainly because the amount of flame retardants used is difficult to control. If too little flame retardant is added, it cannot play a role in improving safety. If too much flame retardant is added, it will affect the battery's electrical properties such as cycle, internal resistance, and rate, especially the electrolyte design of battery chemical reactions and kinetics. There have been no major technological breakthroughs in the industry, causing this technology to remain in the laboratory stage. Compared with safety diaphragm technology, the safety coating on the surface of the pole piece is safer because when the battery fails (lithium-ion battery failure refers to abnormal battery operation due to certain specific material reasons, such as a short circuit in the battery), there is a gap between the diaphragm and the pole piece, and the diaphragm cannot form a stable protection for the pole piece. The safety pole piece coating and the conductive material layer of the pole piece fit tightly together, and the safety performance is better. The pole piece surface safety coating technology is currently the simplest, lowest manufacturing cost, and best technology for improving battery safety.

[0004] The micro-concave coating technology in the surface safety coating technology is a multi-layer coating that is carried out in multiple times. After each layer of coating is completed, it needs to be baked, dried and rolled up, and then re-rolled for the next coating and baking. The process is cumbersome, the coating efficiency is low, and the equipment purchase cost increases. In addition, the foil is baked multiple times to reduce its strength, which makes it easy for the roller to break. The process of using multiple layers of coating to be carried out simultaneously is more advantageous, but the existing technology has a thicker safety coating thickness when multiple layers are coated simultaneously, and the thicker safety coating is likely to affect the energy density of the electrode. Due to the influence of various factors, it is difficult to achieve a thinner safety coating in the industry, especially the double-layer coating technology that is extruded in one time in conjunction with the positive electrode material, which is a process solution that is more difficult to achieve in the industry. Summary of the Invention

[0005] In view of this, the present invention proposes a method for coating a safety layer slurry on a battery positive electrode sheet. The positive electrode sheet safety layer coated by the method of the present invention can make the electrode sheet have a thinner thickness and good safety without affecting the energy density.

[0006] A method for coating a safety layer slurry on a battery positive electrode sheet comprises safety layer slurry preparation, extrusion coating, and baking. The safety layer slurry preparation comprises uniformly mixing a safety solute and a binder, gradually adding a solvent in two or more portions, and performing high-speed and low-speed mixing. The safety layer slurry solution has a solid content of 3-8% and a viscosity of 50-200 cP. The extrusion coating employs a multi-layer extrusion die to simultaneously coat the safety layer slurry and the active material layer slurry, wherein the active material layer slurry is coated between the coated foil and the safety layer slurry. During coating, the distance between the safety layer slurry coating die and the active material layer is 50-130 μm, and the thickness of the safety layer slurry outlet gasket is 0.1-0.3 mm.

[0007] Furthermore, the preparation of the safety layer slurry includes the following steps: adding the binder and the safety solute to the slurry mixing tank, and then stirring in two steps: the first step is to set the stirring speed of the twist paddle to 15-30rpm, the time is 10-45min; the second step is to set the stirring speed of the biaxial dispersion disk to 1000-2000rpm, the time is 15-45min, until the mixture is uniform; adding a solvent to the slurry mixing tank to make the solid content 30-50%, and performing high and low speed combined stirring: setting the twist paddle to stir at a low speed of 15-30rpm for 10-45min, setting the biaxial dispersion disk to stir at a high speed of 2000-3000rpm 90-240min; continue to add solvent to make the solid content of 9-10%, and carry out high-low speed combined stirring: set the twist paddle to stir at a low speed of 15-30rpm for 10-45min, and set the biaxial dispersion disk to stir at a high speed of 2500-3500rpm for 90-150min; continue to add solvent to make the solid content of 3-8%, and carry out high-low speed combined stirring: set the twist paddle to stir at a low speed of 20-30rpm for 30-60min, and set the biaxial dispersion disk to stir at a high speed of 2500-3500rpm for 30-60min; perform vacuum defoaming treatment to obtain the safety layer slurry.

[0008] Furthermore, the weight ratio of the binder and the safety solute in the raw materials for preparing the safety layer slurry is (5-20): (80-95); the binder includes polyvinylidene fluoride, and / or the solvent includes N-methylpyrrolidone, and / or the safety solute includes at least one of boehmite, silica and lithium manganese iron phosphate.

[0009] Furthermore, the raw materials for preparing the active material layer slurry include, by weight, 90-99 parts of active components, 0.2-8 parts of conductive agent and 0.5-3 parts of the binder; the solid content of the active material layer slurry is 68%-78%, the viscosity is 3003-10000 cP, the active component includes lithium nickel cobalt manganese oxide, wherein the molar content of nickel is 83-95%; and / or the conductive agent includes conductive carbon black and carbon nanotubes.

[0010] Furthermore, the opening width of the safety layer slurry outlet gasket in the multi-layer extrusion die head is 2-4 mm smaller than the opening width of the active material layer slurry outlet gasket.

[0011] Furthermore, the chamfer of the safety layer slurry outlet gasket in the opening direction is smaller than that in the depth direction.

[0012] Furthermore, the chamfer of the safety layer slurry outlet gasket in the opening direction is 3*5mm, 3*4mm, 2*4mm or 2*5mm.

[0013] Furthermore, when selecting the safety layer thickness, the safety layer slurry outlet gasket thickness, the solid content of the safety layer slurry solution, and the coating pump speed based on the target value of the surface density of the prepared safety coating, the selection of the above parameters is based on at least one of the following methods: surface density of the safety coating = 1.1009*safety layer thickness-0.5836; surface density of the safety coating = 30.2*safety layer slurry outlet gasket thickness-0.2; surface density of the safety coating = 57.866*solid content of the safety layer slurry solution-0.9698; surface area of ​​the safety coating = 0.1841*coating pump speed+0.7491; the unit of the surface density of the safety coating is g / m2; the unit of the safety layer thickness is μm; the unit of the safety layer slurry outlet gasket thickness is mm; the unit of the solid content of the safety layer slurry solution is 100%; the unit of the coating pump speed is rpm.

[0014] Furthermore, the baking includes the following steps: in the first stage, the temperature is set to 80-100°C and the wind frequency is 10-20Hz for baking; in the second stage, the temperature is set to 80-100°C and the wind frequency is 20-40Hz for baking; in the third stage, the temperature is set to 60-80°C and the wind frequency is 10-20Hz for baking.

[0015] Compared with the prior art, the present invention has the following advantages:

[0016] The coating method of the present invention uses a safety layer slurry with a solid content of 3-8% and a viscosity of 50-200 cP, which can make the baked safety layer thinner and improve coating uniformity and consistency. During coating, the die gap is adjusted to 50-130 μm, and the thickness of the safety layer slurry outlet gasket is 0.1-0.3 mm. When the safety layer is thinner, the safety layer slurry is more widely spread and easier to cover the active material layer. The present invention further preferably adopts a high-speed and low-speed combined stirring preparation process, which can make the prepared safety layer slurry more dispersible, uniform, and consistent. The safety layer coated using this method can make the electrode have a thinner thickness and good safety without affecting the energy density.

[0017] The present invention also provides a battery positive electrode sheet prepared by adopting the above-mentioned method of coating the battery positive electrode sheet safety layer slurry. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] The accompanying drawings, which constitute part of the present invention, are provided to provide a further understanding of the present invention. The exemplary embodiments of the present invention and their descriptions are provided to explain the present invention and do not constitute an undue limitation of the present invention. In the accompanying drawings:

[0019] Figure 1 is a flow chart of the coating method of the present invention;

[0020] Figure 2 Schematic diagram of the coating stage in the coating method of the present invention;

[0021] Figure 3 Schematic diagram of the coating stage in the coating method of the present invention from another perspective;

[0022] Figure 4 is the coating parameter fitting relationship curve in the coating method of the present invention;

[0023] Figure 5 This is a cross-sectional SEM image of the safety layer of Example 1 of the present invention;

[0024] Figure 6 This is a cross-sectional SEM image of the safety layer of Example 2 of the present invention;

[0025] Figure 7 This is a cross-sectional SEM image of the safety layer of Example 3 of the present invention;

[0026] Figure 8 This is a cross-sectional SEM image of the safety layer of Comparative Example 1 of the present invention;

[0027] Figure 9 This is a cross-sectional SEM image of the safety layer of Comparative Example 2 of the present invention;

[0028] Figure 10 This is a surface SEM image of the safety layer of Example 1 of the present invention;

[0029] Figure 11 This is a SEM image of the safety layer surface of Comparative Example 1 of the present invention.

[0030] Description of reference numerals:

[0031] 1. Multi-layer extrusion die; 2. Foil; 3. Coating roller. DETAILED DESCRIPTION

[0032] It should be noted that, in the absence of conflict, the embodiments of the present invention and the features in the embodiments may be combined with each other.

[0033] The experimental methods in the following examples, unless otherwise specified, are conventional methods. The experimental materials used in the following examples, unless otherwise specified, were purchased from conventional biochemical reagent stores. Furthermore, unless otherwise specified in this example, the terms and processes involved in this example should be understood in accordance with general knowledge and conventional methods in the prior art.

[0034] A method for coating a safety layer slurry on a battery positive electrode sheet includes safety layer slurry preparation, extrusion coating, and baking. The safety layer slurry preparation comprises mixing and stirring a safety solute and a binder evenly, gradually adding a solvent in two or more portions and stirring at high and low speeds. The safety layer slurry solution has a solid content of 3-8% and a viscosity of 50-200 cP. The extrusion coating employs a multi-layer extrusion die 1 to simultaneously coat the safety layer slurry and the active material layer slurry, wherein the active material layer slurry is coated between a foil 2 to be coated and the safety layer slurry. During coating, the distance between the safety layer slurry coating die and the active material layer is 50-130 μm, and the thickness of the safety layer slurry outlet gasket is 0.1-0.3 mm.

[0035] The coating method of the present invention uses a safety layer slurry with a solid content of 3-8% and a viscosity of 50-200 cP, which can make the baked safety layer thinner and improve the coating uniformity and consistency. During coating, the gap between the die head and the active material layer, that is, the distance between the safety layer die head and the active material layer slurry coated on the foil 2, is adjusted to 50-130 μm, and the thickness of the safety layer slurry outlet gasket is 0.1-0.3 mm. When the safety layer is thinner, the diffusion of the safety layer slurry is more spread out and it is easier to cover the active material layer. The present invention further adopts high and low speed combined stirring to make the prepared safety layer slurry more dispersible, uniform and consistent. The safety layer coated by the present invention can make the electrode have a thinner thickness and good safety without affecting the energy density.

[0036] Based on the above design ideas, such as Figure 1As shown, the present invention prepares the safety layer slurry and the active material layer slurry in separate tanks in a mixing tank, and puts them into storage tanks for buffering and storage, waiting for coating. After the foil 2 is unwound on the coating roller 3, the buffered active material layer slurry is pumped to the multi-layer extrusion die 1 for coating at the same time. For the convenience of explanation, the present invention takes a double-layer extrusion die as an example, and extrusion dies with other numbers of layers can also be used according to the coating design. The working principle is the same as that of the double-layer extrusion die. Usually, a twin-screw extruder is used to pump the slurry for extrusion coating, so that the safety layer slurry and the active material layer slurry pass through the extrusion die and are evenly coated on the foil 2 at the designed speed and thickness. As shown Figure 2 and Figure 3 As shown, the upper die of the double-layer extrusion die is used to apply the safety layer slurry, while the lower die is used to apply the active material layer slurry. The arrow indicates the unwinding direction of the foil 2, so that the active material layer slurry is applied between the foil 2 being coated and the safety layer slurry. After coating, the slurry is baked in an oven. After drying, the electrode is obtained and reeled.

[0037] Specifically, during the safety layer slurry preparation stage, the preferred safety layer slurry of the present invention comprises a binder and a safety solute in a weight ratio of (5-20):(80-95). The preferred binder comprises polyvinylidene fluoride (PVDF), a highly non-reactive thermoplastic fluoropolymer. The preferred solvent comprises N-methylpyrrolidone (NMP), which is miscible with water in all proportions and has a boiling point of 202°C. During baking, the solvent evaporates, firmly adhering the binder and safety solute to the foil 2. The preferred safety solute comprises at least one of boehmite, silica, and lithium manganese iron phosphate. The safety layer slurry provides insulation, enhancing the safety performance of the battery.

[0038] The raw materials for preparing the active material layer slurry include, by weight, 90-99 parts of active components, 0.2-8 parts of conductive agents, and 0.5-3 parts of binders; the solid content of the active material layer slurry is 68%-78%, and the viscosity is 3003-10000 cP. The active component includes lithium nickel cobalt manganese oxide (NCM), and the conductive agent includes conductive carbon black (SP) and carbon nanotubes (CNT). The preferred mixing ratio is NCM: SP: CNT: PVDF = (90-99): (0.1-5): (0.1-3): (0.5-3). The main function of NCM is to improve the energy density of the battery. Materials with a higher nickel molar content can be selected, such as a nickel molar content of 83-95%.

[0039] The active material layer slurry can be prepared by conventional means in the art, which will not be described in detail here. In order to make the safety layer slurry more uniform, the present invention further recommends the following method for preparing the safety layer slurry.

[0040] During the preparation of the safety layer slurry, in order to avoid agglomeration of the stirred material, a dry mixing process is used, and the material dosage is calculated so that the volume of the prepared slurry is 50%-80% of the slurry mixing tank. Specifically, the following steps may be preferably included: first, an appropriate amount of safety solute and binder are added to the slurry mixing tank, and then stirred evenly in two steps: the first step is to set the stirring speed of the twisted paddle to 15-30rpm and the time to 10-45min; the second step is to set the stirring speed of the dual-axis dispersion disk to 1000-2000rpm and the time to 15-45min.

[0041] After stirring, the container can be left to stand for 30-60 minutes. Because dry stirring is used, opening the container quickly after stirring will cause the powders of the safety solute and binder to splash, resulting in a large amount of dust in the workshop environment. After a period of material sedimentation, solvent is added for stirring. The present invention first stirs the safety solute and binder powders to uniformly disperse the two substances, and then adds the solvent so that the dissolved binder can be evenly coated on the surface of the safety solute, avoiding the generation of agglomerated particles of the safety solute and binder, which is not conducive to the coating of the safety layer.

[0042] After dry mixing, solvent is added to the slurry until the solids content is between 30% and 50%. A combination of high and low speed stirring is then performed, with a twisted paddle set at a low speed of 15-30 rpm for 10-45 minutes, and a dual-shaft dispersing disc set at a high speed of 2000-3000 rpm for 90-240 minutes. A higher solids content makes it easier for the binder to coat the surface with the safe solute after dissolving. The slurry with a high solids content has better dispersibility, which prioritizes the uniform dispersion of the safe solute.

[0043] Continue adding solvent to maintain a solids content within the 9-10% range, then perform a combination of high and low speed stirring: set the twisted paddle at a low speed of 15-30 rpm for 10-45 minutes, and set the dual-shaft disperser at a high speed of 2500-3500 rpm for 90-150 minutes. Continue adding solvent to maintain a solids content of 3-8%, then perform a combination of high and low speed stirring: set the twisted paddle at a low speed of 20-30 rpm for 30-60 minutes, and set the dual-shaft disperser at a high speed of 2500-3500 rpm for 30-60 minutes. Between these stirring cycles, measure the viscosity of the safety layer slurry. By adjusting the amounts of binder and solvent, the viscosity of the safety layer slurry can be adjusted to 50-200 cP, while maintaining a solids content within the 3-8% range. The stirring parameters used above are intended only as a preferred solution; as long as the viscosity and solids content of the safety layer slurry are within the above ranges and the safety solute and binder are evenly dispersed throughout the slurry, the slurry will be suitable. Then, vacuum defoaming treatment is performed, with the preferred vacuum pressure being -90 to -95 kPa, to obtain the safety layer slurry.

[0044] The present invention controls the solid content of the safety layer slurry within the range of 3-8%. Such a low solid content can make the safety layer thinner after baking, while the safety layer slurry extruded during coating is thicker, so that more slurry is extruded through the die during extrusion coating, which is conducive to maintaining the pressure of the extrusion coating die stable, making the safety layer both thinner and more stable from front to back. The viscosity of the safety layer slurry is controlled within the range of 50-200cP. If the viscosity is too low, the slurry is prone to overflow, which affects the uniformity of coating on the electrode. If the slurry viscosity is increased, it is often necessary to increase the proportion of binder. Too much binder will increase the internal resistance of the battery and affect the battery performance. Therefore, the solid content and viscosity of the safety layer slurry of the present invention are conducive to the preparation of a thinner electrode safety layer without affecting the battery performance.

[0045] After the slurry preparation is completed, it is buffered and stored, and then the coating stage can be entered. In the coating stage, the slurry passes through the extrusion coating die under the pressure of the extruder and is coated on the foil 2. A gasket is embedded in the outlet of each extrusion coating die. The thicker the gasket, the thicker the coating, and the larger the gasket opening, the wider the coating width. In order to make the coating thinner and the coating thickness more stable, the present invention designs the safety layer slurry outlet gasket thickness to be 0.1-0.3mm. Because if the gasket thickness is too large, the thickness of the coated safety layer will be larger, affecting the performance of the electrode; if the gasket thickness is too small, the pumping pressure of the extruder will easily cause the gasket to deform and wear more, resulting in a shorter gasket service life. Therefore, a gasket thickness of 0.1-0.3mm is a reasonable setting.

[0046] Compared with the active material layer slurry, the safety layer slurry has a lower viscosity and better fluidity. When the double-layer slurry is extruded, the lower active material layer slurry has a higher viscosity and is not easy to flow, while the upper safety layer slurry is easy to expand laterally. Therefore, the present invention further designs the gasket size, and the opening width of the upper outlet gasket is 2-4mm smaller than the opening width of the lower outlet gasket, so that the safety layer slurry can complete the covering and protection of the lower layer slurry after expansion.

[0047] Because the safety layer slurry has a low viscosity, the present invention further designs the safety layer slurry outlet gasket so that the chamfer in the opening direction is smaller than the depth direction. This prevents excessive slurry thinning or overflow, ensuring smoother and more continuous extrusion of the safety layer slurry. Further preferred chamfer dimensions in the opening direction are 3*5mm, 3*4mm, 2*4mm, or 2*5mm. All of these chamfers can produce a safety layer that meets process requirements.

[0048] Typically, the solid content of the active material layer slurry is 68-78%, and the viscosity is 3003-10000 cP, which is significantly different from the solid content of the safety layer slurry of the present invention. Therefore, the present invention selects a smaller die gap, and the die gap range is 50-130 μm. The smaller die gap is conducive to greater diffusion of the safety layer slurry, so that a lower coating pump speed can be used, so that the safety layer is thin enough and sufficient to cover the active material layer without the problem of uneven coating.

[0049] It is worth noting that the inventors experimentally tested the relationship between the surface density of different safety coatings and the thickness of the safety layer, the thickness of the safety layer slurry outlet gasket, the solid content of the safety layer slurry solution, and the coating pump speed, and conducted single-factor verification respectively. Furthermore, the discrete experimental data were fitted into a relationship curve by the linear regression method, as shown in FIG. Figure 4 As shown. This curve can be used to guide the selection of parameters for preparing the safety layer slurry raw materials and the coating die. Specifically, it includes at least one of the following calculation methods: Safety coating surface density = 1.1009 * safety layer thickness - 0.5836; Safety coating surface density = 30.2 * safety layer slurry outlet gasket thickness - 0.2; Safety coating surface density = 57.866 * safety layer slurry solution solids content - 0.9698; Safety coating surface density = 0.1841 * coating pump speed + 0.7491. The unit of safety coating surface density is g / ㎡; the unit of safety layer thickness is μm; the unit of safety layer slurry outlet gasket thickness is mm; the unit of safety layer slurry solution solids content is 100%; the unit of coating pump speed is rpm. Through the above calculation method, we can find the safety coating suitable for preparing a surface density of 3-6g / ㎡, the required safety layer thickness, the safety layer slurry outlet gasket thickness, the solid content of the safety layer slurry solution and the coating pump speed, which can improve the production preparation efficiency.

[0050] After coating is completed, the slurry can be baked to evaporate the solvent and bond the solid material to the foil 2 to form the electrode. Since the solid content of the safety layer is reduced, the front oven should be set to low wind speed and high temperature during baking. While ensuring that the solvent is baked out, it is necessary to avoid the wind frequency affecting the electrode jitter, which may cause large deviations in the thickness of the safety layer. High wind speed and high temperature are used for baking in the middle. Low wind speed and low temperature are used when shipping out of the oven to avoid large deviations between the oven exit temperature and the external ambient temperature, which may cause the electrode to dry out and crack.

[0051] Baking can be carried out in three stages, specifically including the following steps: in the first stage, the temperature is set to 80-100℃ and the wind frequency is 10-20Hz for baking; in the second stage, the temperature is set to 80-100℃ and the wind frequency is 20-40Hz for baking; in the third stage, the temperature is set to 60-80℃ and the wind frequency is 10-20Hz for baking.

[0052] This invention prepares the safety layer slurry by combining high and low speed stirring, controlling the solid content and viscosity of the safety layer slurry, and rationally designing the spacer and coating parameters to achieve a safety layer thickness of 3-6 μm, a flatness within a ±1 μm range, and an areal density of 3-6 g / m2. This optimization of the slurry preparation process results in a more uniform slurry. Furthermore, improved coating parameters enhance practicality. Furthermore, improved baking parameters make baking more suitable for thinner safety coatings.

[0053] The present invention also provides a battery positive electrode sheet prepared by the above-mentioned method of coating the battery positive electrode sheet safety layer slurry. The battery positive electrode sheet prepared by the above method has a thinner safety layer without affecting the energy density and has good safety performance.

[0054] The specific implementation scheme of the present invention is described in detail below.

[0055] Example 1

[0056] Preparation of safety layer slurry: The raw materials are 2 kg of boehmite, 5.04 kg of polyvinylidene fluoride glue and 40 kg of N-methylpyrrolidone, wherein the solid content of the polyvinylidene fluoride glue is 7%. The above raw materials are added into a 60L stirring tank, and the final solid content of the prepared slurry is 5%.

[0057] Preparation process: First, add appropriate amounts of boehmite and polyvinylidene fluoride adhesive to a mixing tank. Mixing is then carried out in two steps: first, set the paddle stirring speed at 25 rpm for 30 minutes; second, set the biaxial disperser stirring speed at 2500 rpm for 30 minutes. After dry mixing, solvent is added until the solids content reaches 33.41%. High- and low-speed mixing is then performed: the paddle stirring at a low speed of 25 rpm for 25 minutes; then the biaxial disperser stirring at a high speed of 3000 rpm for 210 minutes. After mixing, the paddle is scraped and the solvent is added until the solids content reaches 10%. High- and low-speed mixing is then performed: the paddle stirring at a low speed of 25 rpm for 30 minutes; then the biaxial disperser stirring at a high speed of 3500 rpm for 90 minutes. Solvent is then added until the solids content reaches 5%. The paddle stirring speed is set at a low speed of 25 rpm for 30 minutes; then the biaxial disperser stirring at a high speed of 3500 rpm for 45 minutes. The viscosity of the safety layer slurry was tested to be 102 cP. Then, vacuum defoaming treatment is performed at a pressure of -95 kPa to obtain the safety layer slurry.

[0058] Preparation of active material layer slurry: Use a 60L stirring tank, add NCM 25kg, SP 0.34kg, CNT solution 2.58kg and PVDF glue 4.42kg, NMP 3kg, where the solid content of CNT solution is 5%, the solid content of PVDF glue is 7%, and the final slurry solid content is 72.95%.

[0059] Preparation process: First add PVDF glue and CNT solution, then add SP, and finally add NCM main powder. Stir at high and low speeds after each addition of raw materials; test the viscosity after stirring, add NMP according to the viscosity, and stir at high and low speeds. After stirring, the slurry preparation is completed.

[0060] Coating: When selecting the coating die, select a safety layer gasket opening of 260mm, a chamfer of 3*5mm at the gasket opening, a gasket thickness of 0.15mm, and a coating pump speed of 15rpm. For the safety layer slurry, select a distance of 60µm between the coating die and the active material layer. For the active material layer gasket opening of 263mm, a chamfer of 3*3mm at the gasket opening, a gasket thickness of 0.9mm, and a coating pump speed of 25rpm.

[0061] Baking: The coating oven is 15m long in total, one section of the oven is 3m long, and there are 5 sections of ovens in series, which can carry out three stages of baking with a baking speed of 5m / min. The first stage of baking uses the first and second sections of the oven, and the temperature of the first section is set to 90°C, the wind frequency is 10Hz, and the temperature of the second section is 87°C, and the wind frequency is 15Hz. The second stage of baking uses the third and fourth sections of the oven, the temperature of the third section is 88°C, the wind frequency is 25Hz, and the temperature of the fourth section is 90°C, and the wind frequency is 35Hz. The third stage of baking uses the fifth section of the oven, the temperature of the fifth section is 70°C, and the wind frequency is 15Hz. The safety layer of this embodiment is scanned by an electron microscope, and the SEM image of its cross section is shown as follows. Figure 5 As shown, the surface SEM images are Figure 10 shown.

[0062] Example 2

[0063] Preparation of safety layer slurry: The raw materials are 1 kg of boehmite, 2.52 kg of polyvinylidene fluoride glue and 35 kg of N-methylpyrrolidone, wherein the solid content of the polyvinylidene fluoride glue is 7%. The above raw materials are added into a 60L stirring tank, and the final solid content of the prepared slurry is 3%.

[0064] Preparation process: First, add appropriate amounts of boehmite and polyvinylidene fluoride adhesive to a mixing tank. Mixing is then carried out in two steps: first, set the paddle stirring speed at 25 rpm for 30 minutes; second, set the biaxial disperser stirring speed at 2500 rpm for 30 minutes. After dry mixing, solvent is added to a solids content of 35%. High- and low-speed mixing is then performed: the paddle stirring at a low speed of 25 rpm for 25 minutes; then the biaxial disperser stirring at a high speed of 3000 rpm for 210 minutes. After mixing, the paddle is scraped and the solvent is added to a solids content of 9%. High- and low-speed mixing is then performed: the paddle stirring at a low speed of 25 rpm for 30 minutes; then the biaxial disperser stirring at a high speed of 3500 rpm for 90 minutes. Solvent is then added to a solids content of 3%, and the paddle stirring at a low speed of 25 rpm for 30 minutes; then the biaxial disperser stirring at a high speed of 3500 rpm for 45 minutes. The viscosity of the safety layer slurry was tested to be 65 cP. Then, vacuum defoaming treatment is performed at a pressure of -95 kPa to obtain the safety layer slurry.

[0065] Preparation of active material layer slurry: Use a 60L stirring tank, add NCM 25kg, SP 0.34kg, CNT solution 2.58kg and PVDF glue 4.42kg, NMP 3kg, where the solid content of CNT solution is 5%, the solid content of PVDF glue is 7%, and the final slurry solid content is 72.95%.

[0066] Preparation process: First add PVDF glue and CNT solution, then add SP, and finally add NCM main powder. Stir at high and low speeds after each addition of raw materials; test the viscosity after stirring, add NMP according to the viscosity, and stir at high and low speeds. After stirring, the slurry preparation is completed.

[0067] Coating: When selecting the coating die, select a safety layer gasket opening of 260mm, a chamfer of 3*5mm at the gasket opening, a gasket thickness of 0.1mm, and a coating pump speed of 15rpm. For the safety layer slurry, select a distance of 100µm between the coating die and the active material layer. For the active material layer gasket opening of 263mm, a chamfer of 3*3mm at the gasket opening, a gasket thickness of 0.9mm, and a coating pump speed of 25rpm.

[0068] Baking: The coating oven is 15m long in total, one section of the oven is 3m long, and there are 5 sections of ovens in series, which can carry out three stages of baking with a baking speed of 5m / min. The first stage of baking uses the first and second sections of the oven, and the temperature of the first section is set to 92°C, the wind frequency is 10Hz, and the temperature of the second section is 89°C, and the wind frequency is 15Hz. The second stage of baking uses the third and fourth sections of the oven, the temperature of the third section is 89°C, the wind frequency is 25Hz, and the temperature of the fourth section is 92°C, and the wind frequency is 35Hz. The third stage of baking uses the fifth section of the oven, the temperature of the fifth section is 73°C, and the wind frequency is 15Hz. The safety layer of this embodiment is scanned by an electron microscope, and the SEM image of its cross section is shown as follows. Figure 6 shown.

[0069] Example 3

[0070] Preparation of safety layer slurry: The raw materials are 2 kg of boehmite, 5.04 kg of polyvinylidene fluoride glue and 22.2 kg of N-methylpyrrolidone, wherein the solid content of the polyvinylidene fluoride glue is 7%. The above raw materials are added into a 60L stirring tank, and the final solid content of the prepared slurry is 8%.

[0071] Preparation process: First, add appropriate amounts of boehmite and polyvinylidene fluoride adhesive to a mixing tank. Mixing is then carried out in two steps: first, set the paddle stirring speed at 25 rpm for 30 minutes; second, set the biaxial disperser stirring speed at 2500 rpm for 30 minutes. After dry mixing, solvent is added to a solids content of 30%. High- and low-speed mixing is then performed: the paddle stirring at a low speed of 25 rpm for 25 minutes; then the biaxial disperser stirring at a high speed of 3000 rpm for 210 minutes. After mixing, the paddle is scraped and the solvent is added to a solids content of 10%. High- and low-speed mixing is then performed: the paddle stirring at a low speed of 25 rpm for 30 minutes; then the biaxial disperser stirring at a high speed of 3500 rpm for 90 minutes. Solvent is then added to a solids content of 8%. The paddle stirring speed is set at a low speed of 25 rpm for 30 minutes; then the biaxial disperser stirring speed is set at a high speed of 3500 rpm for 45 minutes. The viscosity of the safety layer slurry was tested to be 187 cP. Then, vacuum defoaming treatment is performed at a pressure of -95 kPa to obtain the safety layer slurry.

[0072] Preparation of active material layer slurry: Use a 60L stirring tank, add NCM 25kg, SP 0.34kg, CNT solution 2.58kg and PVDF glue 4.42kg, NMP 3kg, where the solid content of CNT solution is 5%, the solid content of PVDF glue is 7%, and the final slurry solid content is 72.95%.

[0073] Preparation process: First add PVDF glue and CNT solution, then add SP, and finally add NCM main powder. Stir at high and low speeds after each addition of raw materials; test the viscosity after stirring, add NMP according to the viscosity, and stir at high and low speeds. After stirring, the slurry preparation is completed.

[0074] Coating: When selecting the coating die, select a safety layer gasket opening of 260mm, a chamfer of 3*5mm at the gasket opening, a gasket thickness of 0.3mm, and a coating pump speed of 15rpm. For the safety layer slurry, select a distance of 60µm between the coating die and the active material layer. For the active material layer gasket opening of 263mm, a chamfer of 3*3mm at the gasket opening, a gasket thickness of 0.9mm, and a coating pump speed of 25rpm.

[0075] Baking: The coating oven is 15m long in total, one section of the oven is 3m long, and there are 5 sections of ovens in series, which can carry out three stages of baking with a baking speed of 5m / min. The first stage of baking uses the first and second sections of the oven, and the temperature of the first section is set to 90°C, the wind frequency is 10Hz, and the temperature of the second section is 85°C, and the wind frequency is 15Hz. The second stage of baking uses the third and fourth sections of the oven, the temperature of the third section is 85°C, the wind frequency is 25Hz, and the temperature of the fourth section is 90°C, and the wind frequency is 35Hz. The third stage of baking uses the fifth section of the oven, the temperature of the fifth section is 65°C, and the wind frequency is 15Hz. The safety layer of this embodiment is scanned by an electron microscope, and the SEM image of its cross section is shown as follows. Figure 7 shown.

[0076] Example 4

[0077] Preparation of safety layer slurry: The raw materials are 2 kg of silicon dioxide, 5.04 kg of polyvinylidene fluoride glue and 40 kg of N-methylpyrrolidone, wherein the solid content of the polyvinylidene fluoride glue is 7%. The above raw materials are added into a 60L stirring tank, and the final solid content of the prepared slurry is 5%.

[0078] Preparation process: First, add appropriate amounts of boehmite and polyvinylidene fluoride adhesive to a mixing tank. Mixing is then carried out in two steps: first, using a twisted paddle at 25 rpm for 30 minutes; second, using a biaxial disperser at 2500 rpm for 30 minutes. After dry mixing, solvent is added until the solids content reaches 33.41%. A combination of high and low speed mixing is then performed: the twisted paddle is stirred at a low speed of 25 rpm for 25 minutes, followed by a biaxial disperser at a high speed of 3000 rpm for 210 minutes. After mixing, the paddle is scraped and the solvent is added until the solids content reaches 10%. A combination of high and low speed mixing is then performed: the twisted paddle is stirred at a low speed of 25 rpm for 30 minutes, followed by a biaxial disperser at a high speed of 3500 rpm for 90 minutes. Solvent is then added until the solids content reaches 5%, followed by a low speed of 25 rpm for 30 minutes, followed by a biaxial disperser at a high speed of 3500 rpm for 45 minutes. The viscosity of the safety layer slurry was tested to be 89 cP. Then, vacuum defoaming treatment is performed at a pressure of -95 kPa to obtain the safety layer slurry.

[0079] Preparation of active material layer slurry: Use a 60L stirring tank, add NCM 25kg, SP 0.34kg, CNT solution 2.58kg and PVDF glue 4.42kg, NMP 3kg, where the solid content of CNT solution is 5%, the solid content of PVDF glue is 7%, and the final slurry solid content is 72.95%.

[0080] Preparation process: First add PVDF glue and CNT solution, then add SP, and finally add NCM main powder. Stir at high and low speeds after each addition of raw materials; test the viscosity after stirring, add NMP according to the viscosity, and stir at high and low speeds. After stirring, the slurry preparation is completed.

[0081] Coating: When selecting the coating die, select a safety layer gasket opening of 260mm, a chamfer of 3*5mm at the gasket opening, a gasket thickness of 0.15mm, and a coating pump speed of 15rpm. For the safety layer slurry, select a distance of 65µm between the coating die and the active material layer. For the active material layer gasket opening of 263mm, a chamfer of 3*3mm at the gasket opening, a gasket thickness of 0.9mm, and a coating pump speed of 25rpm.

[0082] Baking: The coating oven is 15 meters long, with each section measuring 3 meters. Five sections are connected in series, enabling three-stage baking at a speed of 5 meters per minute. The first stage of baking utilizes the first and second sections, with the first section set at 90°C and a fan frequency of 10 Hz, and the second section set at 87°C and a fan frequency of 15 Hz. The second stage of baking utilizes the third and fourth sections, with the third section set at 88°C and a fan frequency of 25 Hz, and the fourth section set at 90°C and a fan frequency of 35 Hz. The third stage of baking utilizes the fifth section, with the fifth section set at 70°C and a fan frequency of 15 Hz.

[0083] Comparative Example 1

[0084] The solid content of the safety layer slurry solution is 2.5%, the viscosity is 42CP, the coating gasket thickness is 0.1mm, the die gap is 100um, the active material layer slurry adopts the active material layer slurry of Example 2, and the other preparation parameters, coating method and baking method are the same as Example 2. The safety layer of this comparative example was scanned by electron microscope, and the SEM image of its cross section is shown as follows: Figure 8 As shown, the surface SEM images are Figure 11 shown.

[0085] Comparative Example 2

[0086] The solid content of the safety layer slurry solution is 10%, the viscosity is 215CP, the gasket thickness is 0.35mm, the active material layer slurry adopts the active material layer slurry of Example 3, and the other preparation parameters, coating method and baking method are the same as Example 3. The safety layer of this embodiment is scanned by electron microscope, and the SEM image of its cross section is as follows Figure 9 shown.

[0087] Comparative Example 3

[0088] The safety layer slurry and active material layer slurry of Example 1 were used. During coating, the distance between the coating die of the safety layer slurry and the active material layer was 140 μm. The other preparation parameters, coating method and baking method were the same as those of Example 1.

[0089] The coating effects of the safety layer and the active material layer of the above examples and comparative examples were tested. The test data are shown in Table 1 below:

[0090] Table 1

[0091] plan Example 1 Example 2 Example 3 Example 4 Comparative Example 1 Comparative Example 2 Comparative Example 3 Gasket thickness um 0.15 0.1 0.3 0.15 0.1 0.35 0.15 Slurry solid content% 5 3 8 5 2.5 10 5 Pump speed rpm 15 15 15 15 15 15 15 Safe solute materials Boehmite Boehmite Boehmite Silicon dioxide Boehmite Boehmite Boehmite Viscosity CP 102 65 187 89 42 215 102 Gap um 60 100 60 65 100 60 140 Thickness μm 3.3 2.93 4.9 3.4 0.71 8.16 2.8 Average surface density g / m2 3.2 2.85 4.87 3.6 0.56 7.8 2.68 Surface coating uniformity No missing paint No missing paint No missing paint No missing paint Missing coating No missing paint Missing coating Thickness uniformity um ≤1um ≤1um ≤1um ≤1um ≤1um ≥2um ≥2um Surface coating quality No defects No defects No defects No defects There are bubbles No defects No defects

[0092] Comparison between Example 1 and Example 2 shows that when the solid content is lower than 3%, there will be a significant thinning of the coating thickness, accompanied by the problem of missing coating, and the comparison Figure 11 It can also be seen that the active material layer material leaks out of the boehmite coating, indicating that too low a solid content will cause coating leaks and problems with the active material layer material on the surface.

[0093] Comparison between Comparative Example 2 and Example 3 shows that: the solid content exceeds 8%, and the gasket thickness exceeds 0.3 mm, which will cause the safety layer density to be too thick, resulting in the safety layer thickness being too large, causing a significant increase in the internal resistance of the battery and a decrease in the battery energy density.

[0094] Comparison between Comparative Example 3 and Example 1 shows that under the same parameters, if the die gap is too large, the safety layer slurry will not be able to fully contact the surface of the active material layer slurry, causing leakage of the safety layer and a reduction in coating thickness.

[0095] A needle puncture test was conducted on each of the above embodiments and comparative examples, and a blank sample battery (the blank sample battery is not subjected to the positive electrode safety coating, and the other design parameters are the same as the safety coated electrode, the battery capacity is also the same, and the battery weight is lighter than the battery with the safety coating). A steel needle with a diameter of 5 mm and a length of 100 mm was used at a needle puncture speed of 25 mm / s to test the safety performance of the prepared electrode, and an energy density test was performed for comparison. The test results are shown in Table 2 below:

[0096] Table 2

[0097] plan Example 1 Example 2 Example 3 Example 4 Comparative Example 1 Comparative Example 2 Comparative Example 3 Blank Fire No fire No fire No fire No fire Fire No fire Fire Fire Maximum temperature ℃ 85 93 83 86 548 84 151 756 Acupuncture results pass pass pass pass Fail pass Fail Fail Energy density loss% 1％ 0.85％ 1.6％ 1.1％ 0.6％ 2.5％ 0.75％ 0

[0098] Table 2 shows that the safety coating can help the battery pass the needle puncture. This is mainly because when the needle puncture causes a short circuit between the positive and negative electrodes, the safety coating can block the short circuit through its own insulating electrons, thereby reducing the energy release at the short circuit point, thereby not causing thermal runaway of the entire battery and thus not causing fire problems. However, the increase of the safety coating will lead to an increase in the weight of the electrode, thereby affecting the energy density of the battery, affecting the battery energy density by about 1%. It will not reduce the battery energy density too much and is acceptable in battery design.

[0099] In summary, the present invention improves the parameters of the coating slurry and the coating die head so that the safety layer is thin enough and sufficient to cover the active material layer without causing the problem of uneven coating.

[0100] The preferred embodiments of the present invention are described in detail above. However, the present invention is not limited to the specific details in the above embodiments. Within the technical concept of the present invention, various simple modifications can be made to the technical solution of the present invention, and these simple modifications all fall within the scope of protection of the present invention.

[0101] It should also be noted that the various specific technical features described in the above specific embodiments can be combined in any appropriate manner without contradiction. In order to avoid unnecessary repetition, the present invention will not further describe various possible combinations.

Claims

1. A method for coating a safety layer slurry on a battery positive electrode sheet, comprising preparing the safety layer slurry, extrusion coating, and baking, characterized in that: The safety layer slurry is prepared by mixing the safety solute and the binder and stirring them evenly, adding the solvent gradually in two or more times and stirring at high and low speeds. The solid content of the safety layer slurry solution is 3-8% and the viscosity is 50-200 cP. The extrusion coating uses a multi-layer extrusion die to simultaneously coat the safety layer slurry and the active material layer slurry, wherein the active material layer slurry is coated between the coated foil and the safety layer slurry. During coating, the distance between the coating die of the safety layer slurry and the active material layer is 50-130 μm, and the thickness of the safety layer slurry outlet gasket is 0.1-0.3 mm; The opening width of the safety layer slurry outlet gasket in the multi-layer extrusion die is 2-4 mm smaller than the opening width of the active material layer slurry outlet gasket; The active material layer slurry has a solid content of 68%-78% and a viscosity of 3003-10000 cP.

2. The method for coating a safety layer slurry on a battery positive electrode sheet according to claim 1, characterized in that: The safety layer slurry preparation comprises the following steps: Add the binder and safety solute to the slurry mixing tank, and then stir in two steps: the first step is to set the stirring speed of the twist paddle to 15-30rpm, the time is 10-45min; the second step is to set the stirring speed of the double-shaft disperser to 1000-2000rpm, the time is 15-45min, until the mixture is uniform; Add solvent to the mixing tank to make the solid content 30-50%, and perform high-speed and low-speed mixing: set the twist paddle to stir at a low speed of 15-30 rpm for 10-45 minutes, and set the double-shaft dispersing disk to stir at a high speed of 2000-3000 rpm for 90-240 minutes; Continue to add solvent to make the solid content 9-10%, and stir at high and low speeds: set the twist paddle to stir at a low speed of 15-30 rpm for 10-45 minutes, and set the double-shaft disperser to stir at a high speed of 2500-3500 rpm for 90-150 minutes; Continue to add solvent to make the solid content 3-8%, and stir at high and low speeds: set the twist paddle to stir at a low speed of 20-30 rpm for 30-60 minutes, and set the double-shaft disperser to stir at a high speed of 2500-3500 rpm for 30-60 minutes; The safety layer slurry can be prepared by performing vacuum defoaming treatment.

3. The method for coating a safety layer slurry on a battery positive electrode sheet according to claim 1, characterized in that: The raw materials for preparing the safety layer slurry include a binder and a safety solute in a weight ratio of (5-20): (80-95); the binder includes polyvinylidene fluoride, and / or the solvent includes N-methylpyrrolidone, and / or the safety solute includes at least one of boehmite, silica, and lithium manganese iron phosphate.

4. The method for coating a battery positive electrode safety layer slurry according to claim 1, characterized in that: The raw materials for preparing the active material layer slurry include, by weight, 90-99 parts of active components, 0.2-8 parts of conductive agent and 0.5-3 parts of binder; the active component includes lithium nickel cobalt manganese oxide, wherein the molar content of nickel is 83-95%; and / or the conductive agent includes conductive carbon black and carbon nanotubes.

5. The method for coating a battery positive electrode safety layer slurry according to claim 1, characterized in that: The chamfer angle of the safety layer slurry outlet gasket in the opening direction is smaller than that in the depth direction.

6. The method for coating a battery positive electrode safety layer slurry according to claim 1, characterized in that: The chamfer angle of the safety layer slurry outlet gasket in the opening direction is 3*5mm, 3*4mm, 2*4mm or 2*5mm.

7. The method for coating a safety layer slurry on a battery positive electrode sheet according to claim 1, characterized in that: When selecting the thickness of the safety layer, the thickness of the safety layer slurry outlet gasket, the solid content of the safety layer slurry solution, and the coating pump speed according to the target value of the surface density of the prepared safety coating, the selection of each parameter shall be based on at least one of the following methods: Surface density of safety coating = 1.1009 * thickness of safety layer - 0.5836; Surface density of safety coating = 30.2*thickness of safety layer slurry outlet gasket - 0.2; Surface density of safety coating = 57.866 * solid content of safety layer slurry solution - 0.9698; Areal density of safety coating = 0.1841*coating pump speed + 0.7491; The unit of the surface density of the safety coating is g / ㎡; the unit of the thickness of the safety layer is μm; the unit of the thickness of the safety layer slurry outlet gasket is mm; the unit of the solid content of the safety layer slurry solution is 100%; the unit of the coating pump speed is rpm.

8. The method for coating a battery positive electrode safety layer slurry according to any one of claims 1 to 7, characterized in that: The baking comprises the following steps: in the first stage, the temperature is set to 80-100° C. and the wind frequency is 10-20 Hz for baking; in the second stage, the temperature is set to 80-100° C. and the wind frequency is 20-40 Hz for baking; and in the third stage, the temperature is set to 60-80° C. and the wind frequency is 10-20 Hz for baking.

9. A battery positive electrode sheet prepared by the method for coating a battery positive electrode sheet safety layer slurry according to any one of claims 1 to 8.

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