Positive electrode active material and preparation method thereof, dry-method pole piece and secondary battery

Abstract

The invention relates to a positive electrode active material and a preparation method thereof, a dry-method pole piece and a secondary battery, the positive electrode active material comprises an inner core and a coating layer arranged on at least part of the surface of the inner core, the inner core comprises a ternary positive electrode material, and the coating layer comprises a polydopamine layer, a polyimide layer and a silane coupling agent layer which are laminated along the outward direction of the center of the inner core. The positive electrode active material is provided with the coating layer which is made of a specific material and has a preset layer sequence, the layers realize mutual function support under a specific combination or position relation, and a synergistic effect is achieved, so that the structural stability and the conductivity of the positive electrode active material are better improved, and the service life of the positive electrode active material is prolonged. Therefore, the charge-discharge efficiency, the rate capability and the cycle performance of the obtained secondary battery are improved.

Description

Technical Field

[0001] The present application relates to the field of electrochemical energy storage, and specifically to a positive electrode active material and a preparation method thereof, a dry-process electrode sheet and a secondary battery. Background Art

[0002] Secondary batteries (such as lithium-ion batteries) are highly efficient energy storage devices and are widely used in mobile electronic devices, electric vehicles, and other fields. Ternary cathode materials, with their high energy density and low cost, are becoming one of the preferred cathode materials. However, in practical applications, these materials face challenges such as volume expansion, side reactions at the electrode / electrolyte interface, microcracks, and ion mixing, which reduce the cathode materials' discharge capacity, cycle efficiency, and safety.

[0003] Setting a coating layer on the surface of the positive electrode material can usually enable the ternary positive electrode material to obtain relatively better electrochemical performance. However, many coating materials can provide limited functions in practical applications and may even introduce new defects to the positive electrode material. Therefore, the electrochemical performance of existing positive electrode materials still needs to be improved. Summary of the Invention

[0004] In view of this, the present application provides a positive electrode active material and a preparation method thereof to solve at least one of the above technical problems. In addition, the present application also provides a dry-process electrode and a secondary battery.

[0005] To achieve the above-mentioned objectives, in a first aspect, the present application provides a positive electrode active material, which includes a core and a coating layer provided on at least a portion of the surface of the core, the core includes a ternary positive electrode material, and the coating layer includes a polydopamine layer, a polyimide layer and a silane coupling agent layer stacked in a direction outward from the center of the core.

[0006] In some possible implementations, the chemical formula of the ternary cathode material is Li(Ni x Co y Mn z )O2, where 0.1≤x≤0.8, 0.1≤y≤0.3, 0.1≤z≤0.3, and x+y+z=1.

[0007] In some possible implementations, the thickness ratio of the polydopamine layer, the polyimide layer, and the silane coupling agent layer is (20-80):(100-500):(10-40).

[0008] In some possible implementations, the total thickness of the cladding layer is 130 nm to 620 nm.

[0009] In a second aspect, the present application provides a method for preparing a positive electrode active material, which comprises: sequentially forming a polydopamine layer, a polyimide layer and a silane coupling agent layer on the surface of a lithium-containing ternary positive electrode material to obtain a positive electrode active material; wherein, forming the polydopamine layer comprises: mixing the ternary positive electrode material and dopamine hydrochloride in an alkaline environment, stirring to cause the dopamine hydrochloride to self-polymerize and then coat the surface of the ternary positive electrode material to obtain a first coating; forming the polyimide layer comprises: mixing the first coating and polyamic acid in an organic solvent, stirring to cause the polyamic acid to coat the surface of the first coating to obtain an intermediate, heating the intermediate to imidize the polyamic acid to obtain a second coating; forming the silane coupling agent layer comprises: mixing the second coating and a silane coupling agent in an aqueous solvent, hydrolyzing the silane coupling agent and then coating the surface of the second coating to obtain a positive electrode active material.

[0010] In some possible implementations, the mass ratio of dopamine hydrochloride, polyamic acid and silane coupling agent is (0.3~0.8):(0.8~1.8):(0.5~1.2).

[0011] In some possible implementations, the ratio of the mass of the ternary positive electrode material to the total mass of dopamine hydrochloride, polyamic acid and silane coupling agent is 10:(1.6~3.8).

[0012] In some possible implementations, during the formation of the polydopamine layer, the pH of the alkaline environment is 8 to 10.

[0013] In some possible implementations, during the formation of the polydopamine layer, the stirring time is 1 h to 6 h.

[0014] In some possible implementations, after the polydopamine layer is formed to obtain the first coating, the preparation method further includes: drying the first coating at 80° C. to 120° C. for 3 h to 6 h.

[0015] In some possible implementations, during the formation of the polyimide layer, the stirring time is 2 h to 8 h.

[0016] In some possible implementations, in forming the polyimide layer, heating the intermediate includes: placing the intermediate in a heating device, heating from room temperature to 120°C to 135°C at 1°C / min to 5°C / min, keeping the temperature for 0.5 h to 1 h, heating to 250°C to 350°C at 0.5°C / min to 3°C / min, and keeping the temperature for 1 h to 3 h.

[0017] In some possible implementations, in forming the silane coupling agent layer, the silane coupling agent includes one or both of γ-aminopropyltriethoxysilane and 3-glycidoxypropyltrimethoxysilane.

[0018] In some possible implementations, during the formation of the silane coupling agent layer, the hydrolysis time is 1 h to 4 h.

[0019] In a third aspect, the present application provides a dry-process electrode, which includes a current collector and a dry-process membrane arranged on the surface of the current collector, the dry-process membrane includes a fluoropolymer binder, a conductive agent and an active material, and the active material includes the above-mentioned positive electrode active material or the positive electrode active material prepared by the above-mentioned preparation method.

[0020] In a fourth aspect, the present application provides a secondary battery comprising the above-mentioned dry-process electrode.

[0021] In the positive electrode active material of the present application, the introduction of polyimide improves the electronic conductivity of the positive electrode active material; at the same time, a polydopamine layer is provided on the inner side of the polyimide, and the polydopamine layer is located at the relatively innermost layer, has good adhesion and uniform coating ability, and can form a uniform protective layer on the surface of the core first, and reduce the risk of the coating layer falling off during the cycle. Moreover, the polydopamine layer is formed first on the polyimide layer in the preparation, so it can provide good protection for the core structure during the polyimide layer formation process (such as polyamic acid requires high temperature treatment to undergo imidization, and high temperature may destroy the structure of the core), which is conducive to maintaining the stability of the core structure. properties (such as reducing lithium nickel mixing at high temperatures); secondly, the silane coupling agent layer is located on the outside of the polyimide layer, and the silane coupling agent has good electrical conductivity, which further optimizes the conductive network inside the positive electrode active material. The combination of these two is conducive to exerting better electrical conductivity, thereby compensating for the insufficient electrical conductivity of the polydopamine layer while significantly improving the electrical conductivity of the positive electrode active material; and, the silane coupling agent layer is located in the relatively outermost layer, which is conducive to optimizing the interface compatibility of the positive electrode material, improving the compatibility of the positive electrode active material with other materials in the positive electrode sheet (such as conductive agents and binders), and improving the problem of poor contact interface when the outside of the positive electrode active material is a polymer coating layer. Therefore, the layers of the coating layer of the present application realize mutual functional support under a specific combination or positional relationship, and achieve a synergistic effect, which is conducive to better improving the structural stability and electrical conductivity of the positive electrode active material, thereby helping to improve the charge and discharge efficiency, rate performance and cycle performance of the resulting secondary battery. DETAILED DESCRIPTION

[0022] The embodiments of the present application are described in detail below. The embodiments described below are illustrative and are only used to explain the present application, and should not be understood as limiting the present application. It should be noted that, unless otherwise defined, all technical and scientific terms used herein have the same meanings as those commonly understood by those skilled in the art to which the present application belongs. In the absence of conflict, the embodiments of the present application and the features in the embodiments may be combined with each other. In the following description, many specific details are set forth to facilitate a full understanding of the present application, and the embodiments described are only a part of the embodiments of the present application, not all of the embodiments.

[0023] As a high-performance polymer, polyimide has excellent thermal stability, mechanical strength, and electrical conductivity. Using polyimide as a coating layer for positive electrode active materials has a good protective effect on the positive electrode active materials. However, in related technologies, polyimide is usually formed by imidization of a precursor (such as polyamic acid) through high-temperature treatment. To ensure sufficient imidization to form a polyimide coating layer, the positive electrode active material may need to be subjected to a long-term high-temperature treatment, which may cause damage to the core structure of the positive electrode active material. In addition, polyimide has poor flexibility and is prone to cracking when the volume of the positive electrode active material changes, resulting in uneven coating and affecting the coating effect.

[0024] Based on this, one embodiment of the present application provides an active positive electrode material, which includes a core and a coating layer provided on at least a portion of the surface of the core, the core includes a ternary positive electrode material, and the coating layer includes a polydopamine layer, a polyimide layer and a silane coupling agent layer stacked in a direction from the center of the core toward the outside.

[0025] In the above-mentioned coating layer, the introduction of polyimide improves the electronic conductivity of the positive electrode active material. At the same time, a polydopamine layer is provided on the inner side of the polyimide. The polydopamine layer is located at the relatively innermost layer and has good adhesion and uniform coating ability. It can form a uniform protective layer on the surface of the core first and reduce the risk of the coating layer falling off during the cycle. Moreover, the polydopamine layer is formed on the polyimide layer first in the preparation process. Therefore, it can provide good protection for the core structure during the formation process of the polyimide layer (such as polyamic acid requires high temperature treatment to undergo imidization, and high temperature may destroy the structure of the core), which is beneficial to maintaining the stability of the core structure ( Such as reducing lithium nickel mixing under high temperature); secondly, the silane coupling agent layer is located on the outside of the polyimide layer, and the silane coupling agent has good conductivity, which further optimizes the conductive network inside the positive electrode active material. The combination of the two is conducive to exerting better conductivity, thereby compensating for the insufficient conductivity of the polydopamine layer while significantly improving the conductivity of the positive electrode active material; and, the silane coupling agent layer is located in the relatively outermost layer, which is conducive to optimizing the interface compatibility of the positive electrode material, improving the compatibility of the positive electrode active material with other materials in the positive electrode sheet (such as conductive agents and binders), and improving the problem of poor contact interface when the outside of the positive electrode active material is a polymer coating layer. Therefore, the layers of the coating layer of the present application realize mutual functional support under a specific combination or positional relationship, and achieve a synergistic effect, which is conducive to better improving the structural stability and conductivity of the positive electrode active material, thereby helping to improve the charge and discharge efficiency, rate performance and cycle performance of the resulting secondary battery.

[0026] In some embodiments, the chemical formula of the ternary cathode material is Li(Ni x Co y Mn z)O2, wherein 0.1≤x≤0.8, 0.1≤y≤0.3, 0.1≤z≤0.3, and x+y+z=1. The present application has no special requirements for the nickel content of the ternary positive electrode material. It can be a low-nickel ternary positive electrode material or a high-nickel ternary positive electrode material. In order to protect the structural stability of the ternary positive electrode material, a non-aqueous solvent (such as ethanol) can be appropriately selected to replace water as the solvent of the reactant during the preparation process to reduce the effect of water on the structure of the ternary positive electrode material (such as a high-nickel ternary positive electrode material). In the related art, the high-nickel ternary positive electrode material has a high energy density, but there are problems of cation mixing and more side reactions with the electrolyte. For high-nickel ternary positive electrode materials, the coating layer of the present application is used in high-nickel ternary positive electrode materials, and the polydopamine layer is first formed on the polyimide layer during preparation. Therefore, it can provide good protection for the core structure during the formation of the polyimide layer, which is beneficial to reduce lithium-nickel mixing at high temperatures and maintain the stability of the core structure; and, because the polydopamine layer is located in the relatively innermost layer, it has good adhesion and uniform coating ability. It can itself form a uniform protective layer on the surface of the core first, and reduce the risk of the coating layer falling off during the cycle, which helps to improve the coating quality, thereby helping to reduce the side reactions between the core of the positive electrode material and the electrolyte.

[0027] In some embodiments, the thickness ratio of the polydopamine layer, the polyimide layer, and the silane coupling agent layer is (20-80): (100-500): (10-40). For example, when the thickness of the polydopamine layer is 20 nm to 80 nm, the thickness of the polyimide layer can be 100 nm to 500 nm, and the thickness of the silane coupling agent layer can be 10 nm to 40 nm. The present application also found that controlling the thickness ratio of the polydopamine layer, the polyimide layer, and the silane coupling agent layer in the coating layer within the above range is conducive to better play the effect of mutual support and synergistic enhancement of the functions between the three, thereby helping to better improve the structural stability and conductive properties of the positive electrode active material, thereby further improving the charge and discharge efficiency, rate performance, and cycle performance of the resulting secondary battery.

[0028] In some embodiments, the thickness of the polydopamine layer is 20 nm to 80 nm, the thickness of the polyimide layer is 100 nm to 500 nm, and the thickness of the silane coupling agent layer is 10 nm to 40 nm. For example, the thickness of the polydopamine layer can be 20 nm, 25 nm, 30 nm, 35 nm, 40 nm, 45 nm, 50 nm, 55 nm, 60 nm, 65 nm, 70 nm, 75 nm, 80 nm, or any value within the range formed by any two of the above values. The thickness of the polyimide layer can be 100 nm, 150 nm, 200 nm, 250 nm, 300 nm, 350 nm, 400 nm, 450 nm, 500 nm, or any value within the range formed by any two of the above values. The thickness of the polydopamine layer, polyimide layer and silane coupling agent layer in the coating layer is controlled within the above range, which is conducive to better exerting the effect of mutual support and synergistic enhancement of the functions among the three, thereby helping to better improve the structural stability and conductive properties of the positive electrode active material, thereby further improving the charge and discharge efficiency, rate performance and cycle performance of the resulting secondary battery.

[0029] In some embodiments, the total thickness of the coating layer is 130 nm to 620 nm. For example, the total thickness of the coating layer can be 130 nm, 150 nm, 170 nm, 200 nm, 250 nm, 300 nm, 350 nm, 400 nm, 450 nm, 500 nm, 550 nm, 600 nm, 620 nm, or any value within a range consisting of any two of the foregoing values. Controlling the total thickness of the coating layer within the above range is beneficial for maintaining good conductivity of the positive electrode active material while exerting the function of the coating layer.

[0030] One embodiment of the present application also provides a method for preparing a positive electrode active material, which comprises: sequentially forming a polydopamine layer, a polyimide layer, and a silane coupling agent layer on the surface of a lithium-containing ternary positive electrode material to obtain a positive electrode active material.

[0031] Specifically, the above preparation method comprises: Step 1: forming a polydopamine layer: mixing the ternary cathode material and dopamine hydrochloride in an alkaline environment, stirring to allow the dopamine hydrochloride to self-polymerize and then coat the surface of the ternary cathode material to obtain a first coating.

[0032] In some embodiments, the pH of the alkaline environment is 8 to 10. For example, the pH can be 8, 8.2, 8.4, 8.6, 8.8, 9, 9.2, 9.4, 9.6, 9.8, 10, or any value within a range formed by any two of the foregoing values.

[0033] In some embodiments, the stirring time is 1 h to 6 h. For example, the stirring time can be 1 h, 1.5 h, 2 h, 2.5 h, 3 h, 3.5 h, 4 h, 4.5 h, 5 h, 5.5 h, 6 h, or any value within the range formed by any two of the above values.

[0034] In some embodiments, after obtaining the first coating, the preparation method further includes drying the first coating at 80°C to 120°C for 3 to 6 hours. For example, the drying temperature can be 80°C, 85°C, 90°C, 95°C, 100°C, 105°C, 110°C, 115°C, 120°C, or any value within a range between any two of the foregoing values. The drying time can be 3 hours, 3.5 hours, 4 hours, 4.5 hours, 5 hours, 5.5 hours, 6 hours, or any value within a range between any two of the foregoing values. Drying helps to enhance the bonding strength between the polydopamine layer and the core.

[0035] In some embodiments, a Tris-HCl aqueous solution can be selected as the buffer solution; a Tris-HCl non-aqueous solution containing a non-aqueous solvent as the solvent can also be selected, such as a non-aqueous solvent including ethanol; or a Tris-HCl solution containing a mixture of water and a non-aqueous solvent as the solvent can be selected, such as a volume ratio of ethanol to water of 5:5 to 9:1. Appropriately reducing the water content in the reactants is beneficial to maintaining the structural stability of the ternary cathode material.

[0036] Step 2, forming a polyimide layer: mixing the first coating body and polyamic acid in an organic solvent, stirring to coat the surface of the first coating body with polyamic acid to obtain an intermediate, heating the intermediate to imidize the polyamic acid to obtain a second coating body.

[0037] In some embodiments, the stirring time is 2 h to 8 h. For example, the stirring time can be 2 h, 2.5 h, 3 h, 3.5 h, 4 h, 4.5 h, 5 h, 5.5 h, 6 h, 6.5 h, 7 h, 7.5 h, 8 h, or any value within the range formed by any two of the above values.

[0038] In some embodiments, heating the intermediate comprises: placing the intermediate in a heating apparatus, heating the intermediate from room temperature to 120°C to 135°C at a rate of 1°C / min to 5°C / min, and holding the temperature for 0.5 to 1 hour. For example, the heating rate can be 1°C / min, 2°C / min, 3°C / min, 4°C / min, 5°C / min, or any value within a range formed by any two of the foregoing values. The temperature can be 120°C, 123°C, 126°C, 129°C, 132°C, 135°C, or any value within a range formed by any two of the foregoing values. The holding time can be 0.5 h, 0.6 h, 0.7 h, 0.8 h, 0.9 h, 1 h, or any value within a range formed by any two of the foregoing values. Then, heating the intermediate to 250°C to 350°C at a rate of 0.5 to 3°C / min, and holding the temperature for 1 to 3 hours. For example, the heating rate can be 0.5°C / min, 1°C / min, 1.5°C / min, 2°C / min, 2.5°C / min, 3°C / min, or any value within the range between any two of the above values. The temperature can be 250°C, 270°C, 290°C, 310°C, 330°C, 350°C, or any value within the range between any two of the above values. The holding time can be 1 h, 1.4 h, 1.8 h, 2.2 h, 2.6 h, 3 h, or any value within the range between any two of the above values.

[0039] In the process of forming the polyimide layer, the above-mentioned heating rate is controlled. On the one hand, slow heating is beneficial to reducing the thermal stress concentration caused by the large temperature difference between the surface and the interior of the material, thereby reducing the generation of microcracks or lattice distortion inside the ternary material, and maintaining the chemical stability of the material. For example, high-nickel ternary materials are prone to lithium volatilization and oxygen release at high temperatures, and slow heating can reduce the risk of local overheating; on the other hand, polyimide (polyamic acid solution) needs to be heated to remove solvents (such as NMP, DMF), and slow heating can reduce the rapid volatilization of the solvent to form bubbles or holes, thereby improving the density of the coating and reducing defects. In addition, polyamic acid needs to be dehydrated and closed at high temperature to form polyimide. Slow heating helps to make the imidization reaction more complete, thereby improving the thermal stability and mechanical strength of the coating.

[0040] During the formation of the polyimide layer, a staged heat treatment facilitates precise control of the coating's bonding and morphology. For example, a first heat treatment at 120°C to 135°C helps gently remove residual solvent, preventing residual solvent from loosening the coating. A subsequent heat treatment at 250°C to 350°C helps trigger the imidization reaction and form a stable polyimide cross-linked network. This staged temperature increase solidifies the polyimide layer by layer, forming a continuous coating with uniform thickness. This reduces the risk of localized overburning or cracking caused by a single high-temperature treatment, allowing the formation of a polyimide layer of sufficient thickness (e.g., 100 nm to 500 nm) to provide greater mechanical strength.

[0041] Step three, forming a silane coupling agent layer: mixing the second coating body and the silane coupling agent in an aqueous solvent, hydrolyzing the silane coupling agent and coating the surface of the second coating body to obtain a positive electrode active material.

[0042] In some embodiments, the silane coupling agent includes one or both of γ-aminopropyltriethoxysilane and 3-glycidoxypropyltrimethoxysilane.

[0043] In some embodiments, the hydrolysis time is 1 h to 4 h. For example, the hydrolysis time can be 1 h, 1.5 h, 2 h, 2.5 h, 3 h, 3.5 h, 4 h, or any value within the range formed by any two of the above values.

[0044] In some embodiments, the mass ratio of dopamine hydrochloride, polyamic acid and silane coupling agent is (0.3-0.8): (0.8-1.8): (0.5-1.2). For example, when the amount of dopamine hydrochloride is 0.3 g to 0.8 g, the amount of polyamic acid can be 0.8 g to 1.8 g, and the amount of silane coupling agent can be 0.5 g to 1.2 g. Controlling the mass ratio of dopamine hydrochloride, polyamic acid and silane coupling agent within the above range is conducive to forming a suitable thickness relationship between the polydopamine layer, the polyimide layer and the silane coupling agent layer, thereby helping to enhance the mutual support and synergistic effect of functions between different coating layers.

[0045] In some embodiments, the amount of dopamine hydrochloride is 0.3 g to 0.8 g, the amount of polyamic acid is 0.8 g to 1.8 g, and the amount of silane coupling agent is 0.5 g to 1.2 g. For example, the amount of dopamine hydrochloride can be 0.3 g, 0.35 g, 0.40 g, 0.45 g, 0.50 g, 0.55 g, 0.60 g, 0.65 g, 0.70 g, 0.75 g, 0.8 g, or any value within the range formed by any two of the above values. The amount of polyamic acid can be 0.8 g, 0.9 g, 1 g, 1.1 g, 1.2 g, 1.3 g, 1.4 g, 1.5 g, 1.6 g, 1.7 g, 1.8 g, or any value within the range formed by any two of the above values. The amount of the silane coupling agent can be 0.5 g, 0.55 g, 0.6 g, 0.65 g, 0.7 g, 0.75 g, 0.8 g, 0.85 g, 0.9 g, 0.95 g, 1 g, 1.05 g, 1.1 g, 1.15 g, 1.2 g, or any value within the range formed by any two of the above values. The amount of dopamine hydrochloride, polyamic acid, and silane coupling agent is controlled within the above range, which is conducive to forming a suitable thickness relationship between the polydopamine layer, the polyimide layer, and the silane coupling agent layer, thereby facilitating the mutual support and synergistic effect of the functions between the different coating layers.

[0046] In some embodiments, the ratio of the mass of the ternary positive electrode material to the total mass of dopamine hydrochloride, polyamic acid, and silane coupling agent is 10:(1.6~3.8). For example, when the mass of the ternary positive electrode material is 10 g, the total mass of dopamine hydrochloride, polyamic acid, and silane coupling agent can be 1.6 g to 3.8 g. Controlling the ratio between the mass of the ternary positive electrode material and the total mass of these substances within the above range is conducive to forming a coating layer of appropriate total thickness, so that while playing the role of the coating layer, it is possible to maintain good conductive properties of the positive electrode active material.

[0047] Another embodiment of the present application provides a dry-process electrode, which includes a current collector and a dry-process membrane arranged on the surface of the current collector. The dry-process membrane includes a fluoropolymer binder, a conductive agent and an active material. The active material includes the above-mentioned positive electrode active material or the positive electrode active material prepared by the above-mentioned preparation method.

[0048] In the positive electrode active material of the present application, the silane coupling agent layer is located at the relatively outermost layer, which helps optimize the interfacial compatibility of the positive electrode material and improves the compatibility of the positive electrode active material with other materials in the positive electrode sheet (such as the conductive agent and the binder). This improves the problem of poor contact interface when the outer surface of the positive electrode active material is a polymer coating layer. Therefore, when the positive electrode active material of the present application is used in a dry-process electrode sheet, the silane coupling agent layer also helps improve the interfacial bonding between the positive electrode active material and the fluoropolymer binder, promoting the fiberization of the fluoropolymer binder to form a more uniform network structure after the dry-process membrane is formed, and the components in the dry-process membrane are more evenly mixed.

[0049] Another embodiment of the present application further provides a secondary battery comprising the above-mentioned dry-process electrode, and having good charge and discharge efficiency, rate performance, and cycle performance.

[0050] The present invention will be explained below in conjunction with the embodiments. It will be understood by those skilled in the art that the following examples are only used to explain the present invention and are not to be construed as limiting the present invention. Unless otherwise indicated, the reagents, software, and instruments not specifically described in the following examples are all conventional commercially available products or open source.

[0051] Example 1: (1) A positive electrode active material, the preparation method of which comprises: Step 1: 10 g of ternary active material LiNi 0.8 Co 0.1 Mn 0.1 O2 was dissolved in 30 mL of Tris-HCl ethanol buffer (1 M, pH = 8.5), and then 0.5 g of dopamine hydrochloride powder was added and mixed and stirred for 12 h. Dopamine hydrochloride self-polymerized and coated on the surface of the ternary positive electrode material to obtain the first coating. The first coating was filtered and placed in a vacuum drying oven and dried at 100 °C for 4 h.

[0052] Step 2: Carry out a condensation reaction between a dibasic acid anhydride and a diamine at a temperature not higher than 0°C for at least 8 hours to obtain polyamic acid; take 1.2 g of polyamic acid and mix it with an appropriate amount of NMP solvent to obtain a polyamic acid solution; mix the obtained polyamic acid solution with the first coating, stir it thoroughly so that the polyamic acid is coated on the surface of the dried first coating to obtain an intermediate, place the intermediate in a high-temperature oven for thermal imidization, heat it from room temperature to 135°C over 1 hour, keep it warm for 30 minutes, then heat it to 300°C over 2 hours, and keep it warm for 1 hour to obtain a second coating.

[0053] Step 3: Dissolve 0.8 g of γ-aminopropyltriethoxysilane (KH550) in 30 mL of ethanol / water mixed solvent (V / V=4:1) to obtain a KH550 solution. Mix the obtained KH550 solution with the second coating, form a uniform silane coupling agent layer on the surface through hydrolysis and condensation reactions, and dry it to obtain the positive electrode active material.

[0054] (2) A dry electrode, the preparation method of which comprises: The positive electrode active material, conductive agent and fluorine-containing polymer binder obtained in (1) are mixed at a ratio of 95:2:3 at 10°C, and then rapidly heated and subjected to high-speed shearing and fiberization to obtain a mixed powder. The mixed powder is rolled into a film under high temperature conditions using a roller press, and finally compounded with a carbon-coated current collector to obtain a dry-process electrode sheet, which is a positive electrode sheet.

[0055] (3) A secondary battery, the preparation method of which comprises: The dry-process electrode sheet (positive electrode sheet), lithium sheet (negative electrode sheet) and separator obtained in (2) are assembled to obtain an electrode assembly, and an electrolyte is injected to obtain a secondary battery, which is a button battery.

[0056] Example 2: The difference from Example 1 is that 10 g of ternary active material, 0.3 g of dopamine hydrochloride powder, 0.8 g of polyamic acid, and 0.5 g of KH550 are used.

[0057] Example 3: The difference from Example 1 is that 10 g of ternary active material, 0.8 g of dopamine hydrochloride powder, 1.8 g of polyamic acid, and 1.2 g of KH550 are used.

[0058] Example 4: The difference from Example 1 is that 0.2 g of dopamine hydrochloride powder is used.

[0059] Example 5: The difference from Example 1 is that 0.6 g of polyamic acid is used.

[0060] Example 6: The difference from Example 1 is that 0.3 g of KH550 is used.

[0061] Example 7: The difference from Example 3 is that 1.5 g of dopamine hydrochloride powder is used.

[0062] Example 8: The difference from Example 3 is that 3.0 g of polyamic acid is used.

[0063] Example 9: The difference from Example 3 is that 2.5 g of KH550 is used.

[0064] Comparative Example 1: The difference from Example 1 is that in (1), the first step is adjusted to polyamic acid coating and thermal imidization, the second step is adjusted to polydopamine coating, and the third step remains unchanged. That is, in the obtained positive electrode active material, the polyimide layer, the polydopamine layer, and the silane coupling agent layer are coated in sequence from the center of the core toward the outside.

[0065] Comparative Example 2: The difference from Example 1 is that in (1), the first step is adjusted to hydrolysis and coating of the silane coupling agent, the second step remains unchanged, and the third step is adjusted to polydopamine coating. That is, in the obtained positive electrode active material, the silane coupling agent layer, the polyimide layer, and the polydopamine layer are coated in order from the center of the core toward the outside.

[0066] The present application conducted a thickness test on the coating layer of the positive electrode active materials of Examples 1-9 and Comparative Examples 1-2. The specific method includes: analyzing the cross section of the positive electrode active material by transmission electron microscopy (JEOL-2100, accelerating voltage 200 kV) and measuring the thickness of the coating layer; The present application also conducted a conductivity test on the dry-process electrodes of the above-mentioned Examples 1-9 and Comparative Examples 1-2. The specific method includes: measuring by a four-probe method (RTS-9 resistance meter, pressure 5 MPa).

[0067] The present application also conducted electrochemical performance tests on the secondary batteries of Examples 1-9 and Comparative Examples 1-2. The specific method includes: conducting performance tests on the secondary batteries at a voltage of 2.8 V to 4.3 V and 25°C.

[0068] Please refer to Table 1 and Table 2 for the above test results.

[0069] Table 1. Test results of the positive electrode active material coating layer structure, dry electrode conductivity, and secondary battery electrochemical performance of Examples 1-9 of the present application Table 2. Test results of the positive electrode active material coating layer structure, dry electrode conductivity and secondary battery electrochemical performance of Comparative Examples 1-2 of this application The positive electrode active materials of Examples 1-9 of the present application have coating layers with a preset positional relationship, namely, the polydopamine layer, the polyimide layer and the silane coupling agent layer. First, the introduction of polyimide improves the electronic conductivity of the positive electrode active material. The polydopamine layer is arranged on the inner side of the polyimide. The polydopamine layer is located at the relatively innermost layer and has good adhesion and uniform coating ability. It can form a uniform protective layer on the surface of the core first and reduce the risk of the coating layer falling off during the cycle. Moreover, the polydopamine layer is formed first on the polyimide layer in preparation, so it can be the core structure in the process of forming the polyimide layer (such as polyamic acid needs high temperature treatment to undergo imidization, and high temperature may destroy the structure of the core). Providing good protection is beneficial to maintaining the stability of the core structure (such as reducing lithium nickel mixing at high temperatures); secondly, the silane coupling agent layer is located on the outside of the polyimide layer, and the silane coupling agent has good electrical conductivity, which further optimizes the conductive network inside the positive electrode active material. The combination of the two is beneficial to exerting better electrical conductivity, thereby compensating for the insufficient electrical conductivity of the polydopamine layer while significantly improving the electrical conductivity of the positive electrode active material; the silane coupling agent layer is located in the relatively outermost layer, which is also beneficial to optimizing the interface compatibility of the positive electrode material, improving the compatibility of the positive electrode active material with other materials in the positive electrode sheet (such as conductive agents and binders), and improving the problem of poor contact interface when the outer side of the positive electrode active material is a polymer coating layer. Therefore, the layers of the positive electrode active material coating layer of Examples 1-9 of the present application achieve mutual functional support under a specific combination or positional relationship, and achieve a synergistic effect, which is beneficial to better improve the structural stability and electrical conductivity of the positive electrode active material, thereby helping to improve the charge and discharge efficiency, rate performance and cycle performance of the resulting secondary battery.

[0070] Among them, Examples 1-3 control the mass ratio of the coating material during preparation to fall within a preset range, thereby obtaining a positive electrode active material whose thickness ratio of each coating layer meets the preset range. At this time, the functional mutual support and synergistic effect between the polydopamine layer, the polyimide layer and the silane coupling agent layer are better exerted, which is conducive to better improving the structural stability and conductive properties of the positive electrode active material, and further improving the charge and discharge efficiency, rate performance and cycle performance of the obtained secondary battery.

[0071] Compared with Examples 1-9, the coating layer sequence of the positive electrode active material in Comparative Example 1 does not satisfy the preset position relationship, the polydopamine layer is not located on the inner side of the polyimide layer, the core structure of the positive electrode active material is not stable enough, and the polyimide layer is easy to fall off, the coating quality is poor, and the performance of the resulting secondary battery is not as good as that of the embodiment.

[0072] Compared with Examples 1-9, the coating layer sequence of the positive electrode active material of Comparative Example 2 also does not meet the preset positional relationship. The silane coupling agent layer is not located on the outside of the polyimide layer. The conductive effect of the positive electrode active material is poor, and when prepared into a dry-process electrode, the mixing uniformity is insufficient, and the performance of the resulting secondary battery is not as good as that of the embodiment.

[0073] The above embodiments are only used to illustrate the technical solutions of the present application and are not intended to limit the present application. Although the present application has been described in detail with reference to the above preferred embodiments, those skilled in the art should understand that modifications or equivalent replacements of the technical solutions of the present application should not depart from the spirit and scope of the technical solutions of the present application.

Claims

1. A positive electrode active material, characterized in that The positive electrode active material includes a core and a coating layer arranged on at least a portion of the surface of the core, the core includes a ternary positive electrode material, and the coating layer includes a polydopamine layer, a polyimide layer and a silane coupling agent layer stacked in an outward direction from the center of the core.

2. The positive electrode active material according to claim 1, wherein The chemical formula of the ternary cathode material is Li(Ni x Co y Mn z )O2, where 0.1≤x≤0.8, 0.1≤y≤0.3, 0.1≤z≤0.3, and x+y+z=1.

3. The positive electrode active material according to claim 1, wherein The positive electrode active material further satisfies at least one of the following conditions: (1) The thickness ratio of the polydopamine layer, the polyimide layer, and the silane coupling agent layer is (20-80):(100-500):(10-40); (2) The total thickness of the coating layer is 130 nm to 620 nm.

4. A method for preparing a positive electrode active material, characterized in that: The preparation method comprises: A polydopamine layer, a polyimide layer and a silane coupling agent layer are sequentially formed on the surface of the lithium-containing ternary positive electrode material to obtain the positive electrode active material; wherein, Forming the polydopamine layer includes: mixing the ternary cathode material and dopamine hydrochloride in an alkaline environment, stirring to allow the dopamine hydrochloride to self-polymerize and then coat the surface of the ternary cathode material to obtain a first coating; Forming the polyimide layer includes: mixing the first coating body and polyamic acid in an organic solvent, stirring to coat the polyamic acid on the surface of the first coating body to obtain an intermediate, and heating the intermediate to imidize the polyamic acid to obtain a second coating body; Forming the silane coupling agent layer includes: mixing the second coating body and a silane coupling agent in an aqueous solvent, hydrolyzing the silane coupling agent and coating the surface of the second coating body to obtain the positive electrode active material.

5. The preparation method according to claim 4, wherein The preparation method satisfies at least one of the following conditions: (1) The mass ratio of the dopamine hydrochloride, the polyamic acid and the silane coupling agent is (0.3-0.8):(0.8-1.8):(0.5-1.2); (2) The ratio of the mass of the ternary positive electrode material to the total mass of the dopamine hydrochloride, the polyamic acid and the silane coupling agent is 10:(1.6~3.8).

6. The preparation method according to claim 4, wherein The formation of the polydopamine layer further satisfies at least one of the following conditions: (1) The pH of the alkaline environment is 8 to 10; (2) The stirring time is 1 h to 6 h; (3) After obtaining the first coating body, the preparation method further includes: drying the first coating body at 80° C. to 120° C. for 3 h to 6 h.

7. The preparation method according to claim 4, wherein The formation of the polyimide layer further satisfies at least one of the following conditions: (1) The stirring time is 2 h to 8 h; (2) Heating the intermediate comprises: placing the intermediate in a heating device, heating the intermediate from room temperature to 120°C to 135°C at a rate of 1°C / min to 5°C / min, and keeping the temperature therefor for 0.5 h to 1 h, heating the intermediate to 250°C to 350°C at a rate of 0.5°C / min to 3°C / min, and keeping the temperature therefor for 1 h to 3 h.

8. The preparation method according to claim 4, wherein The formation of the silane coupling agent layer further satisfies at least one of the following conditions: (1) The silane coupling agent includes one or both of γ-aminopropyltriethoxysilane and 3-glycidoxypropyltrimethoxysilane; (2) The hydrolysis time is 1 h to 4 h.

9. A dry electrode, characterized in that: The dry-process electrode sheet includes a current collector and a dry-process membrane arranged on the surface of the current collector, the dry-process membrane includes a fluoropolymer binder, a conductive agent and a positive electrode active material, and the positive electrode active material includes the positive electrode active material according to any one of claims 1 to 3 or the positive electrode active material prepared by the preparation method according to any one of claims 4 to 8.

10. A secondary battery, characterized in that: Including the dry electrode as described in claim 9.

Images