Dry-method positive electrode material and preparation method thereof, dry-method positive plate and preparation method thereof, and secondary battery

Abstract

The invention provides a dry-method positive electrode material and a preparation method thereof, a dry-method positive plate and a preparation method thereof, and a secondary battery. The preparation method of the dry-process positive electrode material comprises the following steps: carrying out first mixing on a positive electrode active material, a first binder and a first conductive agent to obtain an intermediate material; performing second mixing on the intermediate material, a second binder and a second conductive agent to obtain a dry-method positive electrode material; the first binder is a non-fibrillated binder; the second binder is a binder capable of being fibrillated; the first conductive agent is a first granular conductive agent, and the second conductive agent is selected from at least one of a second granular conductive agent, a one-dimensional conductive agent and a two-dimensional conductive agent. According to the invention, based on a dry process, by selecting the components in the first mixing process and the second mixing process, the purposes of constructing an efficient three-dimensional conductive network, reducing the porosity and improving the compaction density of the material are achieved.

Description

Technical Field

[0001] This invention relates to the field of secondary batteries, and more specifically, to a dry-process cathode material and its preparation method, a dry-process cathode sheet and its preparation method, and a secondary battery. Background Technology

[0002] In the global wave towards carbon neutrality, power batteries and energy storage systems are the core pillars. As the "heart" of the battery, the manufacturing process of the electrode directly determines the battery's performance, cost, and environmental attributes. The traditional wet process, which involves slurry preparation and coating, has many drawbacks, including: (1) high cost of NMP and high energy consumption for solvent evaporation; (2) NMP is listed as a restricted chemical, which is highly harmful to the environment and human health, and has high usage requirements; (3) due to solvent evaporation, the thickness of the wet electrode is limited, which restricts the improvement of battery energy density. The dry process, which abandons the limitations of high cost, high energy consumption, low environmental protection, and low energy of the traditional wet process, has become one of the hot topics in the current battery field.

[0003] Under the dual pressure of energy revolution and environmental protection requirements, dry process breaks free from the constraints of solvent system and caters to the future development direction of lithium battery. However, dry process differs from traditional wet coating in terms of material contact and interface characteristics, resulting in higher internal resistance in batteries. There are four reasons for this: (1) The conductive network is not well constructed. During the dry mixing process, the conductive agent cannot achieve uniform dispersion comparable to that of wet slurry; (2) Dry electrode has higher porosity, resulting in poor electrolyte wetting effect; (3) The dry film and foil are not firmly bonded, and the large electrode rebound will increase the contact resistance; (4) PTFE is used as a fibrillated binder with a high content, increasing the proportion of the insulating network.

[0004] In other words, although the dry process theoretically offers a greener and more economical way to manufacture batteries, in practical applications, it still faces significant challenges in areas such as conductive network construction, porosity control, composite technology, binder content, and process parameter control.

[0005] Therefore, how to prepare dry cathode materials with lower resistivity based on dry process is one of the important technical problems that need to be solved in this field. Summary of the Invention

[0006] The main objective of this invention is to provide a dry cathode material and its preparation method, a dry cathode sheet and its preparation method, and a secondary battery, so as to solve the problem that it is difficult to prepare low resistivity cathode materials based on dry processes in the prior art.

[0007] To achieve the above objectives, a first aspect of the present invention provides a method for preparing a dry cathode material, comprising: step S1, wherein a cathode active material, a first binder, and a first conductive agent are first mixed to obtain an intermediate material; step S2, wherein the intermediate material, a second binder, and a second conductive agent are second mixed to obtain a dry cathode material; wherein the first binder is a non-fibrillated binder, and the polymer backbone of the non-fibrillated binder is a rigid chain; the second binder is a fibrillable binder, and the polymer backbone of the fibrillable binder is a flexible chain; the first conductive agent is a first particulate conductive agent, and the second conductive agent is selected from at least one of a second particulate conductive agent, a one-dimensional conductive agent, and a two-dimensional conductive agent.

[0008] The aforementioned dry preparation method, through careful selection of components in the first and second mixing processes, achieves the goals of constructing a highly efficient three-dimensional conductive network, reducing porosity, and increasing material compaction density. In the first mixing process, a rigid polymer (i.e., a non-fibrillated binder) is selected as the first binder to pre-form a connecting network. This allows the positive electrode active material and the first conductive agent to form an initial agglomerated structure without significantly increasing the hardness of the mixture. Secondly, a three-dimensional conductive network is formed: the first conductive agent is a particulate conductive agent, which, with the help of the non-fibrillated binder, enables the active material and the first conductive agent to form an initial close contact. Although this close contact is partially weakened in the second mixing step, the remaining close contact is still better than the poor contact problem caused by conventional mixing methods. The introduction of the second conductive agent compensates for the limitations of the first conductive agent, forming a more complex three-dimensional conductive network, significantly enhancing the conductivity of the electrode and the electrochemical reaction rate. Finally, in the second mixing step, the flexible polymer (i.e., the fibrillating binder) undergoes a certain degree of fibrillation at high temperature, which can serve as a component of the obtained dry cathode material. In subsequent applications (such as film formation), it forms a more robust fiber network, strengthens the adhesion between cathode material particles, reduces the springback of the prepared electrode sheet during the compaction process, and maintains a reasonable porosity, which is beneficial for the full wetting of the electrolyte and ion transport.

[0009] More importantly, by adjusting the mixing sequence and combining it with the selection of binder and conductive agent types at different stages, this invention effectively controls the agglomeration degree and pore distribution of the resulting dry-process cathode material. This sequence of using rigid binders followed by flexible binders allows the cathode material to maintain good dispersion and conductive network construction in the initial stage, while gaining additional structural stability during molding and compaction. The synergistic effect of both ensures the conductivity of the resulting cathode material and improves its mechanical strength after film formation, ultimately resulting in superior electrochemical activity.

[0010] Furthermore, to improve the compatibility between the components and obtain a dry-process cathode material with higher electrochemical activity, preferably: the first binder is selected from one or more of polyacrylic acid, polyacrylate, styrene-butadiene rubber, sodium carboxymethyl cellulose, polyacrylonitrile, polyimide, polyvinylidene fluoride, polypropylene, and polyethylene; and / or, the polymer monomer of the second binder includes tetrafluoroethylene; and / or, the first particulate conductive agent and the second particulate conductive agent are each independently selected from one or more of conductive carbon black, conductive graphite, acetylene black, and Ketjen black; and / or, the one-dimensional conductive agent is selected from carbon nanotubes, One or more of carbon fibers, carbon nanotubes, and metal fibers; and / or, the two-dimensional conductive agent is graphene and / or conductive graphite; preferably, the first binder is selected from one or more of polyvinylidene fluoride, styrene-butadiene rubber, and sodium carboxymethyl cellulose, more preferably polyvinylidene fluoride; and / or, the second binder is selected from one or more of polytetrafluoroethylene, ethylene-tetrafluoroethylene copolymer, and hexafluoropropylene-tetrafluoroethylene copolymer, more preferably polytetrafluoroethylene; and / or, the first conductive agent is conductive carbon black; and / or, the second conductive agent is selected from at least two of conductive carbon black, carbon nanotubes, and graphene.

[0011] Further, in step S1, the first mixing is carried out in a paddle mixer, which includes the following stages performed sequentially: A1 stage with a blade tip linear velocity of 20m / s to 50m / s and a time of 10min to 30min; A2 stage with a blade tip linear velocity of 40m / s to 60m / s and a time of 3min to 30min; and A3 stage with a blade tip linear velocity of 10m / s to 30m / s and a time of 10min to 30min; and / or, in step S2, the second mixing is carried out in a paddle mixer, which includes the following stages performed sequentially: B1 stage with a blade tip linear velocity of 10m / s to 20m / s and a time of 3min to 10min; and B2 stage with a blade tip linear velocity of 30m / s to 50m / s and a time of 3min to 10min. The optimal blade tip linear velocity and time at each of the above stages can work together to improve the performance of dry cathode materials, more effectively construct material bonding and conductive networks from the microscopic to the macroscopic level, and thus more significantly reduce the internal resistance of the obtained dry cathode materials and improve their electrochemical activity.

[0012] Furthermore, in the first mixing process, stage A1 is carried out at 25±2℃, stage A2 at 100℃~150℃, and stage A3 at 20℃~50℃; and / or, in the second mixing process, stage B1 is carried out at 5℃~15℃, and stage B2 at 80℃~100℃. Optimizing the temperatures of each stage allows the intermediate material to achieve optimization throughout the entire process from basic dispersion to deep bonding and structural stability in the first mixing process, thereby better strengthening the bonding in the second mixing process and ultimately forming a denser composite structure with lower electrical resistance.

[0013] Furthermore, in order to maintain a high energy density while promoting the resulting dry-process cathode material to exhibit lower battery internal resistance and better electrochemical cycle performance, preferably: the weight ratio of the first binder to the second binder is 1:(1~3); and / or, the weight ratio of the first conductive agent to the second conductive agent is (0.3~0.7):(0.5~1.2); and / or, based on 100% of the weight of the dry-process cathode material, the content of the cathode active material is 96.5%~98.5%, the total content of the first conductive agent and the second conductive agent is 0.5%~1.5%, and the total content of the first binder and the second binder is 1.0%~2.0%.

[0014] A second aspect of the present invention provides a dry cathode material prepared by the above-described method for preparing dry cathode materials. Thanks to the meticulous control of the mixing sequence and component types in the above preparation method, the resulting dry cathode material achieves a comprehensive improvement in electrochemical performance, including high energy density, good cycle stability, and fast charging capability, and especially possesses lower internal resistance.

[0015] A third aspect of the present invention provides a dry-process cathode sheet, comprising a cathode active material, wherein the cathode active material is the aforementioned dry-process cathode material. Due to the use of the aforementioned dry-process cathode material with low internal resistance, high compaction, and superior mechanical properties, the porosity of the resulting dry-process cathode sheet can be controlled within a reasonable range, which is beneficial for rapid electrolyte wetting and ensures high compaction density, thereby improving the energy density and cycle stability of the battery.

[0016] A fourth aspect of the present invention provides a method for preparing a dry-process positive electrode sheet, comprising: step R1, preparing the above-mentioned dry-process positive electrode material into a self-supporting film; and step R2, combining the self-supporting film with a current collector to form a dry-process positive electrode sheet. By first preparing the above-mentioned dry-process positive electrode material into a self-supporting film, and then combining it as an active layer with a current collector, not only can a positive electrode sheet with superior performance be obtained, but the use of solvents and the resulting energy consumption and pollution problems in traditional wet processes can also be avoided.

[0017] Furthermore, to promote more efficient compaction of the positive electrode active material and more stable composite with the current collector, thereby obtaining a positive electrode sheet and secondary battery with lower internal resistance, preferably: step R1 includes: preparing a self-supporting membrane from the dry-process positive electrode material at 80℃~120℃ using a multi-stage differential speed rolling method with a rolling pressure of 200kg / cm~800kg / cm and a speed ratio greater than 1; step R2 includes: composite the self-supporting membrane with the current collector at 100℃~140℃ using a rolling pressure of 300kg / cm~1000kg / cm to form a dry-process positive electrode sheet; preferably, the thickness of the self-supporting membrane is 90μm~200μm, the tensile strength is 0.1MPa~1MPa, and the electrode resistivity is 30Ω. cm~90Ω cm.

[0018] A fifth aspect of the present invention provides a secondary battery comprising a positive electrode, wherein the positive electrode is a dry-processed positive electrode as described above; or, the positive electrode is prepared by the method described above for preparing a dry-processed positive electrode. Due to the significant advantages of the dry-processed positive electrode obtained by the present invention in terms of processing performance, electrochemical performance, and environmental friendliness, the secondary battery in which it is located can achieve higher energy density, faster charge / discharge rates, and longer cycle life.

[0019] By applying the technical solution of this invention, based on a dry process, and through careful selection of components in the first and second mixing processes, the invention achieves the goals of constructing a highly efficient three-dimensional conductive network, reducing porosity, and increasing material compaction density. This results in a significant improvement in battery electrochemical performance and a reduction in battery internal resistance. The resulting dry-process cathode material exhibits better processing performance and pore control, which helps enhance electrolyte wetting and further optimizes the electrochemical reaction kinetics of the battery. Attached Figure Description

[0020] The accompanying drawings, which form part of this application, are used to provide a further understanding of the invention. The illustrative embodiments of the invention and their descriptions are used to explain the invention and do not constitute an undue limitation of the invention. In the drawings:

[0021] Figure 1 The results are shown in the scanning electron microscope (SEM) characterization of the dry cathode material obtained in Example 1 of this invention. Detailed Implementation

[0022] It should be noted that, unless otherwise specified, the embodiments and features described in this application can be combined with each other. The present invention will now be described in detail with reference to the embodiments.

[0023] As described in the background section, existing technologies suffer from the difficulty of preparing low-resistivity cathode materials using dry processes. To address this technical problem, a first aspect of the present invention provides a method for preparing a dry cathode material, comprising: step S1, mixing a cathode active material, a first binder, and a first conductive agent to obtain an intermediate material; step S2, mixing the intermediate material, a second binder, and a second conductive agent to obtain the dry cathode material; wherein the first binder is a non-fibrillated binder, and the polymer backbone of the non-fibrillated binder is a rigid chain; the second binder is a fibrillable binder, and the polymer backbone of the fibrillable binder is a flexible chain; the first conductive agent is a first particulate conductive agent, and the second conductive agent is selected from at least one of a second particulate conductive agent, a one-dimensional conductive agent, and a two-dimensional conductive agent.

[0024] The aforementioned dry preparation method, through careful selection of components in the first and second mixing processes, achieves the goals of constructing a highly efficient three-dimensional conductive network, reducing porosity, and increasing the compaction density of the material. Specifically:

[0025] First, an initial flexible framework is constructed: In the first mixing process, a rigid polymer (i.e., a non-fibrillated binder) is selected as the first binder to pre-form a connecting network. This allows the positive electrode active material and the first conductive agent to form an initial aggregated structure without significantly increasing the hardness of the mixture. The existence of this framework is crucial for subsequent processing steps because it provides sufficient deformation space to ensure that the material does not become over-hardened during the second mixing step, thus maintaining the uniformity of the internal structure. Second, a three-dimensional conductive network is formed: The first conductive agent is a particulate conductive agent, which, with the help of the non-fibrillated binder, allows the active material to form an initial close contact with the first conductive agent. Although this close contact is partially weakened during the second mixing step, the remaining close contact is still better than the poor contact caused by conventional mixing methods. The introduction of the second conductive agent can compensate for the limitations of the first conductive agent, forming a more complex three-dimensional conductive network, significantly enhancing the conductivity of the electrode and the electrochemical reaction rate. Finally, in the second mixing step, the flexible polymer (i.e., the fibrillating binder) undergoes a certain degree of fibrillation at high temperature, which can serve as a component of the obtained dry cathode material. In subsequent applications (such as film formation), it forms a more robust fiber network, strengthens the adhesion between cathode material particles, reduces the springback of the prepared electrode sheet during the compaction process, and maintains a reasonable porosity, which is beneficial for the full wetting of the electrolyte and ion transport.

[0026] More importantly, by adjusting the mixing sequence and combining it with the selection of binder and conductive agent types at different stages, this invention effectively controls the agglomeration degree and pore distribution of the resulting dry-process cathode material. This sequence of using rigid binders followed by flexible binders allows the cathode material to maintain good dispersion and conductive network construction in the initial stage, while gaining additional structural stability during molding and compaction. The synergistic effect of both ensures the conductivity of the resulting cathode material and improves its mechanical strength after film formation, ultimately resulting in superior electrochemical activity.

[0027] Furthermore, to improve the compatibility between the components and obtain a dry-process cathode material with higher electrochemical activity, preferably: the first binder is selected from one or more of polyacrylic acid, polyacrylate, styrene-butadiene rubber, sodium carboxymethyl cellulose, polyacrylonitrile, polyimide, polyvinylidene fluoride, polypropylene, and polyethylene; and / or, the polymer monomer of the second binder includes tetrafluoroethylene; and / or, the first particulate conductive agent and the second particulate conductive agent are each independently selected from one or more of conductive carbon black, conductive graphite, acetylene black, and Ketjen black; and / or, the one-dimensional conductive agent is selected from one or more of carbon whiskers, carbon fibers, carbon nanotubes, and metal fibers; and / or, the two-dimensional conductive agent is graphene and / or conductive graphite. Specifically, the metal fiber may be aluminum fiber and / or aluminum / polyester composite fiber.

[0028] Based on the aforementioned component types, a further preferred embodiment is selected from one or more of polyvinylidene fluoride, styrene-butadiene rubber, and sodium carboxymethyl cellulose; and / or, the second binder is selected from one or more of polytetrafluoroethylene, ethylene-tetrafluoroethylene copolymer, and hexafluoropropylene-tetrafluoroethylene copolymer; and / or, the first conductive agent is conductive carbon black; and / or, the second conductive agent is selected from at least two of conductive carbon black, carbon nanotubes, and graphene. In this preferred embodiment, the first binder can better aggregate the first conductive agent around the active particles, while the second binder can form a denser microstructure, strengthening the physical connection of the electrode material and reducing the interfacial resistance without significantly increasing the overall resistance. The selection of two conductive agents can further enhance the three-dimensionality of the established conductive network, and at the same time, through different electron transport paths, further reduce the electron migration resistance of the resulting dry-process active material.

[0029] In several more typical embodiments, the first binder is preferably polyvinylidene fluoride (PVDF), and the second binder is polytetrafluoroethylene (PTFE). PVDF can more effectively adhere the first conductive agent to the vicinity of the active particles, while the fibrillation ability of PTFE further strengthens the bonding of the components through the fiber network at high temperatures, thus more effectively improving the compaction density and areal density of the resulting dry-process cathode material.

[0030] Furthermore, in order to construct a multi-layered stable conductive network and further reduce the internal resistance of the obtained dry cathode material, the second conductive agent is preferably a composition formed by conductive carbon black and carbon nanotubes in a weight ratio of (1~2):1; or, the second conductive agent is a composition formed by conductive carbon black and graphene in a weight ratio of (1~2):1.

[0031] In practical applications, the positive electrode active material can be selected from one or more of lithium iron phosphate, lithium manganese iron phosphate, ternary cathode materials, and lithium-rich manganese-based materials. Among these, the above-mentioned preparation method has the most significant effect on reducing the internal resistance of ternary cathode materials.

[0032] In step S1, the first mixing is preferably carried out in a paddle mixer, comprising the following stages in sequence: A1 stage with a blade tip linear velocity of 20 m / s to 50 m / s and a time of 10 min to 30 min; A2 stage with a blade tip linear velocity of 40 m / s to 60 m / s and a time of 3 min to 30 min; and A3 stage with a blade tip linear velocity of 10 m / s to 30 m / s and a time of 10 min to 30 min. The mild premixing conditions in stage A1 facilitate more uniform dispersion of the positive electrode active material, the first conductive agent, and the first binder, forming more stable preliminary agglomerates. The high-speed mixing in stage A2 accelerates the interaction between materials through high energy input, further enhancing the encapsulation effect of the binder and the dispersion effect of the conductive agent, thereby further improving the conductivity of the mixture. The slow mixing in stage A3 helps to better maintain the structural stability of the mixture, reducing particle damage and conductive network disruption caused by excessive mixing.

[0033] In step S2, the second mixing is preferably carried out in a paddle mixer, comprising: a B1 stage with a blade tip linear velocity of 10 m / s to 20 m / s and a time of 3 min to 10 min, and a B2 stage with a blade tip linear velocity of 30 m / s to 50 m / s and a time of 3 min to 10 min. The premixing in the B1 stage helps to achieve a more uniform distribution of the second conductive agent and the second binder in the first mixture, reducing material oxidation or structural damage. The subsequent fiberization process in the B2 stage, through high-speed stirring, more effectively promotes the fibrillation of the second binder, forming an additional fiber network, further reducing the porosity of the resulting dry-process cathode material and optimizing its electrochemical performance.

[0034] The stirring parameters of the two mixing processes obtained above can work together to improve the performance of dry cathode materials, more effectively construct material bonding and conductive networks from the micro to the macro level, and thus more significantly reduce the internal resistance of the obtained dry cathode materials and improve their electrochemical activity.

[0035] Based on the above mixing conditions, in order to optimize the entire process of intermediate materials from basic dispersion to deep bonding and structural stability in the first mixing process, and further enhance bonding in the second mixing process to ultimately form a more compact composite structure with lower resistance, the following further optimizations are made: In the first mixing process, stage A1 is carried out at 25±2℃, stage A2 is carried out at 100℃~150℃, and stage A3 is carried out at 20℃~50℃; and / or, in the second mixing process, stage B1 is carried out at 5℃~15℃, and stage B2 is carried out at 80℃~100℃.

[0036] For the first mixing stage, in stage A1, the room temperature of 25±2℃ promotes a gentle initial contact between the active material and the first conductive agent. Simultaneously, the rigid polymer maintains good fluidity at this temperature, further facilitating the formation of a thin and uniform adhesive layer together with the conductive agent. In stage A2, the temperature rises to 100℃~150℃. Within this range, the non-fibrillated binder slightly melts, achieving better fluidity and adhesion, enabling more effective encapsulation of the positive electrode active material and forming more stable microstructure aggregates. Simultaneously, the high temperature of 100℃~150℃ helps promote the micro-melting of the first binder and more thorough physical contact between conductive agent particles, constructing a more uniform and stable conductive network. Finally, in stage A3, lowering the temperature to 20℃~50℃ helps solidify the previously formed basic structure, reducing internal stress accumulation caused by rapid binder solidification during cooling. This allows the components in the intermediate material to slowly solidify after forming aggregates, ultimately improving the microstructural stability and uniformity of the final dry-process positive electrode material. Regarding the second mixing stage, the low-temperature premixing conditions in stage B1 help to better maintain the original morphology of the second conductive agent and the second binder, reducing premature fiberization of the second binder and thus minimizing agglomeration. In the subsequent stage B2, the temperature is increased to 80℃~100℃. At this temperature, the second binder, i.e., the flexible polymer, begins to fiberize, forming a more stable fibrous network structure and enhancing the physical connections between electrode materials. Simultaneously, this temperature range also better promotes the further dispersion of the second conductive agent, forming a more continuous conductive path, thereby significantly improving the electrochemical performance of the resulting dry-process cathode material.

[0037] In several typical embodiments, the weight ratio of the first binder to the second binder is 1:(1~3), more preferably 1:(1.5~2). In the above-mentioned preferred and more preferred binder ratios, rigid polymers can promote uniform coverage between the positive electrode active material and the first conductive agent, constructing a more stable primary conductive network. Flexible polymers can also better form a more robust microstructure network through the fiberization process, enhancing the mechanical properties of the resulting dry-process positive electrode material and increasing its compaction density.

[0038] In several other typical embodiments, the weight ratio of the first conductive agent to the second conductive agent is (0.3~0.7):(0.5~1.2), more preferably 1:(1.5~2). These preferred and more preferred conductive agent ratios not only enhance the electron transport path within the resulting dry-process cathode material but also more effectively construct a multi-layered conductive network, thereby significantly reducing the internal resistance of the resulting dry-process cathode material and improving its electrochemical performance.

[0039] Furthermore, in order to maintain a high energy density while enabling the resulting dry-process cathode material to exhibit lower battery internal resistance and better electrochemical cycle performance, it is preferable that, based on 100% by weight of the dry-process cathode material, the content of cathode active material is 96.5% to 98.5%, the total content of the first conductive agent and the second conductive agent is 0.5% to 1.5%, and the total content of the first binder and the second binder is 1.0% to 2.0%.

[0040] A second aspect of the present invention provides a dry cathode material prepared by the above-described method for preparing dry cathode materials. Thanks to the meticulous control of the mixing sequence and component types in the above preparation method, the resulting dry cathode material achieves a comprehensive improvement in electrochemical performance, including high energy density, good cycle stability, and fast charging capability, and especially possesses lower internal resistance.

[0041] It should be noted that, due to the specific nature of the materials field and the limitations of existing testing and characterization methods, it is difficult to perform a comprehensive quantitative characterization of the dry-process cathode material obtained above. However, performance test results show that when the dry-process cathode material obtained in this invention is used in a battery system, both the cathode sheet and the battery exhibit lower internal resistance and higher electrical performance.

[0042] A third aspect of the present invention provides a dry-process cathode sheet, comprising a cathode active material, wherein the cathode active material is the aforementioned dry-process cathode material. Due to the use of the aforementioned dry-process cathode material with low internal resistance, high compaction, and superior mechanical properties, the porosity of the resulting dry-process cathode sheet can be controlled within a reasonable range, which is beneficial for rapid electrolyte wetting and ensures high compaction density, thereby improving the energy density and cycle stability of the battery.

[0043] A fourth aspect of the present invention provides a method for preparing a dry-process positive electrode sheet, comprising: step R1, preparing the above-mentioned dry-process positive electrode material into a self-supporting film; and step R2, combining the self-supporting film with a current collector to form a dry-process positive electrode sheet. By first preparing the above-mentioned dry-process positive electrode material into a self-supporting film, and then combining it as an active layer with a current collector, not only can a positive electrode sheet with superior performance be obtained, but the use of solvents and the resulting energy consumption and pollution problems in traditional wet processes can also be avoided.

[0044] In several typical embodiments, in order to promote more efficient compaction of the positive electrode active material and more stable composite with the current collector, thereby obtaining a positive electrode sheet and a secondary battery with lower internal resistance, preferred step R1 includes: preparing a self-supporting membrane of dry-process positive electrode material by multi-stage differential rolling at 80°C to 120°C with a rolling pressure of 200 kg / cm to 800 kg / cm (more preferably 400 kg / cm to 600 kg / cm) and a speed ratio greater than 1; preferred step R2 includes: composite the self-supporting membrane and the current collector into a dry-process positive electrode sheet at 100°C to 140°C with a rolling pressure of 300 kg / cm to 1000 kg / cm (more preferably 600 kg / cm to 800 kg / cm) (the speed ratio during composite is generally 1).

[0045] In several preferred embodiments, the thickness of the self-supporting film is 90 μm to 200 μm, the tensile strength is 0.1 MPa to 1 MPa, and the electrode resistivity is 30 Ω. cm~90Ω cm.

[0046] In practical applications, the multi-stage differential roller presses used in steps R1 and R2 are all four-stage differential roller presses, and their speed ratios can be adjusted according to actual needs.

[0047] A fifth aspect of the present invention provides a secondary battery comprising a positive electrode, wherein the positive electrode is a dry-processed positive electrode as described above; or, the positive electrode is prepared by the method described above for preparing a dry-processed positive electrode. Due to the significant advantages of the dry-processed positive electrode obtained by the present invention in terms of processing performance, electrochemical performance, and environmental friendliness, the secondary battery in which it is located can achieve higher energy density, faster charge / discharge rates, and longer cycle life.

[0048] The present application will be further described in detail below with reference to specific embodiments, which should not be construed as limiting the scope of protection claimed in the present application.

[0049] Unless otherwise defined, all technical terms used herein have the same meaning as commonly understood by those skilled in the art. The technical terms used herein are for the purpose of describing particular embodiments only and are not intended to limit the scope of the invention.

[0050] Example 1

[0051] A method for preparing a dry cathode material:

[0052] (S1) Ternary cathode material (NCM622), conductive carbon black (SP), and polyvinylidene fluoride (PVDF) are premixed at room temperature (25±2℃) (A1 stage, blade tip linear velocity 40m / s, time 30min). Based on the total weight of the final dry-process cathode material being 100%, the ternary cathode material accounts for 97% of the total mass, the currently added PVDF (i.e., the first binder) accounts for 0.5% of the total mass, and the currently added SP (i.e., the first conductive agent) accounts for 0.5% of the total mass.

[0053] The tank is heated to 100°C and stirred at high speed at this temperature (A2 stage, blade tip linear velocity 50 m / s, time 30 min). Then, the temperature is lowered to 45°C and stirred slowly at low speed at this temperature (A3 stage, blade tip linear velocity 10 m / s, time 30 min) to obtain intermediate material.

[0054] (S2) Continue to reduce the temperature of the tank to 13°C, and at this temperature add conductive carbon black + carbon nanotubes (i.e. SP + CNT, with a weight ratio of 1:1) as the second conductive agent and polytetrafluoroethylene (PTFE) as the second binder, and premix (B1 stage, blade tip linear velocity 20m / s, time 10min).

[0055] The temperature was then raised to 90°C and fibroped at this temperature (B2 stage, tip linear velocity 40 m / s, time 5 min), after which the dry cathode material was obtained. The mass percentage of the second conductive agent added is 1%, and the mass percentage of the second binder PTFE is 1%.

[0056] That is, based on the weight of the final dry-process cathode material as 100%, the content of ternary cathode active material is 97%, the total content of conductive agent is 1.5% (of which the weight ratio of the first conductive agent to the second conductive agent is 1:2), and the total content of binder is 1.5% (of which the weight ratio of the first binder to the second binder is 1:2).

[0057] The SEM characterization results of the obtained dry-process cathode material are shown in Figure 1 .according to Figure 1 It can be seen that the particle distribution of the obtained dry-process cathode material is uniform.

[0058] A method for preparing a dry-process positive electrode:

[0059] (R1) At 100℃, with a rolling pressure of 555 kg / cm and a film-forming speed ratio of 1.5~3.5 (4-stage differential rollers, with the speed ratios of rollers 1 to 5 being 3.5-3-1.6-1.5 respectively), the above-obtained dry cathode material is prepared into a self-supporting film with a thickness of 95 μm.

[0060] (R2) The obtained self-supporting membrane is laminated with carbon-coated foil and then composited into a dry positive electrode at 140°C with a rolling pressure of 666 kg / cm and a speed ratio of 1.

[0061] In the above process, the film-forming equipment and the laminating equipment used are all 4-stage differential speed roller pressing equipment.

[0062] Example 2

[0063] A method for preparing a dry cathode material:

[0064] The only difference between this embodiment and Embodiment 1 is that an equal weight of SP+graphene (SP:graphene = 2:1, by weight) is used to replace the second conductive agent SP+CNT; and the temperature conditions of the A2 stage are changed to 150°C.

[0065] A method for preparing a dry positive electrode sheet: consistent with Example 1.

[0066] Example 3

[0067] A method for preparing a dry cathode material:

[0068] The only difference between this embodiment and Embodiment 1 is that an equal weight of SP is used to replace the second conductive agent SP+CNT; and the temperature conditions of stage B2 are changed to 100°C.

[0069] A method for preparing a dry positive electrode sheet: consistent with Example 1.

[0070] Example 4

[0071] A method for preparing a dry cathode material:

[0072] The only difference between this embodiment and Embodiment 1 is that the amount of the first conductive agent is changed to 0.7%, the amount of the second conductive agent is changed to 0.8%, and the time of the B1 stage is changed to 3 minutes.

[0073] Based on the weight of the final dry-process cathode material as 100%, the content of ternary cathode active material is 97%, the total content of conductive agent is 1.5% (of which the weight ratio of the first conductive agent to the second conductive agent is 0.7:0.8), and the total content of binder is 1.5% (of which the weight ratio of the first binder to the second binder is 1:2).

[0074] A method for preparing a dry positive electrode sheet: consistent with Example 1.

[0075] Example 5

[0076] A method for preparing a dry cathode material:

[0077] The only difference between this embodiment and Embodiment 1 is that the amount of the first conductive agent is changed to 0.7%, the amount of the second conductive agent is changed to 0.5%, and the amount of the second adhesive is changed to 1.3%; at the same time, the temperature conditions of stage B2 are changed to 80°C.

[0078] Based on the weight of the final dry-process cathode material as 100%, the content of ternary cathode active material is 97%, the total content of conductive agent is 1.2% (of which the weight ratio of the first conductive agent to the second conductive agent is 0.7:0.5), and the total content of binder is 1.8% (of which the weight ratio of the first binder to the second binder is 0.5:1.3).

[0079] A method for preparing a dry positive electrode sheet: consistent with Example 1.

[0080] Example 6

[0081] A method for preparing a dry cathode material:

[0082] The only difference between this embodiment and Embodiment 1 is that the amount of the first conductive agent is changed to 0.3%, and the amount of the second conductive agent is changed to 1.2%; at the same time, the temperature conditions of the A2 stage are changed to 130°C.

[0083] Based on the weight of the final dry-process cathode material as 100%, the content of ternary cathode active material is 97%, the total content of conductive agent is 1.5% (of which the weight ratio of the first conductive agent to the second conductive agent is 1:4), and the total content of binder is 1.5% (of which the weight ratio of the first binder to the second binder is 1:2).

[0084] A method for preparing a dry positive electrode sheet: consistent with Example 1.

[0085] Example 7

[0086] A method for preparing a dry cathode material:

[0087] The only difference between this embodiment and Embodiment 1 is that the tip linear velocity in stage A1 is changed to 60 m / s and the time is changed to 5 min; the tip linear velocity in stage A2 is changed to 80 m / s and the time is changed to 1 min; and the tip linear velocity in stage A3 is changed to 5 m / s and the time is changed to 40 min.

[0088] A method for preparing a dry positive electrode sheet: consistent with Example 1.

[0089] Example 8

[0090] A method for preparing a dry cathode material:

[0091] The only difference between this embodiment and Embodiment 1 is that the tip linear velocity in stage A1 is changed to 10 m / s and the time is changed to 40 min; the tip linear velocity in stage A2 is changed to 30 m / s and the time is changed to 40 min; and the tip linear velocity in stage A3 is changed to 40 m / s and the time is changed to 5 min.

[0092] A method for preparing a dry positive electrode sheet: consistent with Example 1.

[0093] Example 9

[0094] A method for preparing a dry cathode material:

[0095] The only difference between this embodiment and Embodiment 1 is that: stage A1 is changed to be performed at 50°C; stage A2 is changed to be performed at 80°C; and stage A3 is changed to be performed at 10°C.

[0096] A method for preparing a dry positive electrode sheet: consistent with Example 1.

[0097] Example 10

[0098] A method for preparing a dry cathode material:

[0099] The only difference between this embodiment and Embodiment 1 is that: stage A1 is changed to be performed at 10°C; stage A2 is changed to be performed at 160°C; and stage A3 is changed to be performed at 60°C.

[0100] A method for preparing a dry positive electrode sheet: consistent with Example 1.

[0101] Example 11

[0102] A method for preparing a dry cathode material:

[0103] The only difference between this embodiment and Embodiment 1 is that the tip linear velocity in stage B1 is changed to 5 m / s and the time is changed to 20 min; the tip linear velocity in stage B2 is changed to 60 m / s and the time is changed to 1 min.

[0104] A method for preparing a dry positive electrode sheet: consistent with Example 1.

[0105] Example 12

[0106] A method for preparing a dry cathode material:

[0107] The only difference between this embodiment and Embodiment 1 is that the tip linear velocity in stage B1 is changed to 30 m / s and the time is changed to 1 min; the tip linear velocity in stage B2 is changed to 20 m / s and the time is changed to 15 min.

[0108] A method for preparing a dry positive electrode sheet: consistent with Example 1.

[0109] Example 13

[0110] A method for preparing a dry cathode material:

[0111] The only difference between this embodiment and Embodiment 1 is that stage B1 is changed to be performed at 20°C, and stage B2 is changed to be performed at 120°C.

[0112] A method for preparing a dry positive electrode sheet: consistent with Example 1.

[0113] Example 14

[0114] A method for preparing a dry cathode material: consistent with Example 1.

[0115] A method for preparing a dry-process positive electrode:

[0116] The only difference between this embodiment and Embodiment 1 is that in step (R1), the rolling pressure is changed to 200 kg / cm; and in step (R2), the rolling pressure is changed to 300 kg / cm.

[0117] Example 15

[0118] A method for preparing a dry cathode material: consistent with Example 1.

[0119] A method for preparing a dry-process positive electrode:

[0120] The only difference between this embodiment and Embodiment 1 is that in step (R1), the rolling pressure is changed to 800 kg / cm; and in step (R2), the rolling pressure is changed to 1000 kg / cm.

[0121] Comparative Example 1

[0122] A method for preparing a cathode material:

[0123] The only difference between this comparative example and Example 1 is that steps (S1) and (S2) were not performed; instead, the active material, the first conductive agent, and the first binder were directly mixed to obtain the positive electrode material powder composition.

[0124] A method for preparing a wet-process positive electrode sheet: N-methylpyrrolidone (NMP) is used as a solvent, and the above-mentioned positive electrode material powder composition is added thereto to obtain a slurry with a solid content of 65%. (The final step is to prepare the slurry at 350 g / m³.) 2 Wet cathode sheets are obtained by coating with surface density.

[0125] Comparative Example 2

[0126] A method for preparing a cathode material:

[0127] The only difference between this comparative example and Example 1 is that steps (S1) and (S2) were not performed; instead, the active material, the second conductive agent, and the second binder were directly mixed to obtain a positive electrode material powder composition.

[0128] A method for preparing a wet positive electrode: consistent with Comparative Example 1.

[0129] Comparative Example 3

[0130] A method for preparing a cathode material:

[0131] The only difference between this comparative example and Example 1 is that steps (S1) and (S2) were not performed. Instead, the active material, the first conductive agent, the first binder, the second conductive agent, and the second binder were directly mixed to obtain a positive electrode material powder composition.

[0132] A method for preparing a wet positive electrode: consistent with Comparative Example 1.

[0133] Comparative Example 4

[0134] A method for preparing a dry cathode material:

[0135] The only difference between this comparative example and Example 1 is that, in step (S1), an equal weight of PTFE is used instead of PVDF as the first adhesive.

[0136] A method for preparing a dry positive electrode sheet: consistent with Example 1.

[0137] Comparative Example 5

[0138] A method for preparing a dry cathode material:

[0139] The only difference between this comparative example and Example 1 is that, in step (S2), an equal weight of PVDF is used instead of PTFE as the second binder.

[0140] A method for preparing a dry positive electrode sheet: consistent with Example 1.

[0141] Test methods

[0142] The powder resistivity of the positive electrode material sample and the electrode resistivity of the self-supporting film sample were obtained by testing under a pressure of 200 MPa using PRCD2100 (IEST Yuaneng Technology).

[0143] Tensile strength of self-supporting membrane samples: obtained according to GB / T 36363-2018.

[0144] Battery sample preparation and performance testing: Graphite electrode sheets were prepared using conventional wet process as negative electrodes, and positive electrode samples obtained from each example and comparative example were used as positive electrodes. The electrolyte system of 1M LiPF6, EC:EMC:DMC=2:3:2, and 1%wt VC was used as electrolyte. The samples were assembled into 20Ah soft-pack battery samples. After formation and capacity testing, the internal resistance of the battery samples was tested.

[0145] The above tests were performed on the cathode material samples, self-supporting film samples, and battery samples obtained in each embodiment and comparative example, and the results are shown in Table 1.

[0146] Table 1

[0147]

[0148] As can be seen from the above description, compared with the comparative examples, the embodiments of the present invention achieve the preparation of high-performance dry cathode materials. Thanks to the meticulous mixing sequence and component type control in the above preparation method, the obtained dry cathode material achieves a comprehensive improvement in electrochemical performance, including low internal resistance, high energy density, good cycle stability, and fast charging capability. When used as a positive electrode active material to prepare a self-supporting film and then a dry cathode sheet, the obtained dry cathode sheet exhibits significant advantages in processing performance, electrochemical performance, and environmental friendliness, ultimately resulting in a secondary battery with better electrical performance.

[0149] Specifically, in each embodiment:

[0150] Comparing Example 3 with Examples 1 and 2, it can be seen that, for the second conductive agent, it is preferable to select two or more of conductive carbon black, carbon nanotubes and graphene, which can further enhance the three-dimensionality of the established conductive network, and at the same time, through different electron transport paths, further reduce the electron migration resistance of the obtained dry active material.

[0151] Comparing Examples 5 and 6 with Examples 1 and 4, it is evident that when the weight ratios of the first binder to the second binder and the first conductive agent to the second conductive agent are optimized, the rigid polymer can promote uniform coverage between the positive electrode active material and the first conductive agent, constructing a more stable primary conductive network. Meanwhile, the flexible polymer can better form a more robust microstructure network through the fibrosis process, enhancing the mechanical properties of the resulting dry-process positive electrode material and increasing its compaction density. Simultaneously, it can more effectively construct a multi-layered conductive network, thereby significantly reducing the internal resistance of the resulting dry-process positive electrode material and improving its electrochemical performance.

[0152] Comparing Examples 7 and 8 with Example 1, it can be seen that by optimizing the tip linear velocity and time in stages A1, A2, and A3 respectively, it is possible to achieve more uniform dispersion between the positive electrode active material, the first conductive agent, and the first binder, forming a more stable preliminary aggregate, thereby further enhancing the encapsulation effect of the binder and the dispersion effect of the conductive agent, and ultimately further improving the conductivity of the mixture.

[0153] Comparing Examples 11 and 12 with Example 1, it can be seen that optimizing the tip linear velocity and time for stages B1 and B2 can help the second conductive agent and the second binder to be more evenly distributed in the first mixture, while more effectively promoting the fibrillation of the second binder to form an additional fiber network, further reducing the porosity of the obtained dry cathode material and optimizing its electrochemical performance.

[0154] Comparing Examples 9, 10, and 13 with Example 1, it can be seen that by optimizing the temperatures of stages A1, A2, A3, B1, and B2, the entire process from basic dispersion to deep bonding and then to structural stability can be optimized, thereby better strengthening the bonding in the second mixing process and ultimately forming a more compact composite structure with lower resistance.

[0155] Comparing Examples 14 and 15 with Example 1, it can be seen that by optimizing the rolling pressure during the film formation and composite process in the preparation of the positive electrode sheet, the positive electrode active material can be compacted more efficiently and composited more stably with the current collector, thereby obtaining a positive electrode sheet and a secondary battery with lower internal resistance.

[0156] It should be noted that the terms "first," "second," etc., used in the specification and claims of this application are used to distinguish similar objects and are not necessarily used to describe a specific order or sequence. It should be understood that such terms can be used interchangeably where appropriate so that the embodiments of this application described herein can be implemented, for example, in a sequence other than those described herein.

[0157] The above description is merely a preferred embodiment of the present invention and is not intended to limit the invention. Various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for preparing a dry cathode material, characterized in that, include: Step S1: The positive electrode active material, the first binder, and the first conductive agent are mixed to obtain an intermediate material; In step S2, the intermediate material, the second binder, and the second conductive agent are mixed in a second process to obtain the dry-process cathode material. The first adhesive is a non-fibrillated adhesive, and the polymer backbone of the non-fibrillated adhesive is a rigid chain; the second adhesive is a fibrillable adhesive, and the polymer backbone of the fibrillable adhesive is a flexible chain. The first conductive agent is a first particulate conductive agent, and the second conductive agent is selected from at least one of a second particulate conductive agent, a one-dimensional conductive agent, and a two-dimensional conductive agent.

2. The method for preparing the dry cathode material according to claim 1, characterized in that, The first adhesive is selected from one or more of polyacrylic acid compounds, polyacrylates, styrene-butadiene rubber, sodium carboxymethyl cellulose, polyacrylonitrile compounds, polyimide compounds, polyvinylidene fluoride, polypropylene, and polyethylene; and / or, The polymer monomer of the second adhesive includes tetrafluoroethylene; and / or, The first granular conductive agent and the second granular conductive agent are each independently selected from one or more of conductive carbon black, conductive graphite, acetylene black, and Ketjen black; and / or, The one-dimensional conductive agent is selected from one or more of carbon whiskers, carbon fibers, carbon nanotubes, and metal fibers; and / or, The two-dimensional conductive agent is graphene and / or conductive graphite; Preferably, the first binder is selected from one or more of polyvinylidene fluoride, styrene-butadiene rubber, and sodium carboxymethyl cellulose, more preferably polyvinylidene fluoride; and / or, the second binder is selected from one or more of polytetrafluoroethylene, ethylene-tetrafluoroethylene copolymer, and hexafluoropropylene-tetrafluoroethylene copolymer, more preferably polytetrafluoroethylene; and / or, the first conductive agent is conductive carbon black; and / or, the second conductive agent is selected from at least two of conductive carbon black, carbon nanotubes, and graphene.

3. The method for preparing the dry cathode material according to claim 2, characterized in that, In step S1, the first mixing is carried out in a paddle mixer, which includes the following stages performed sequentially: A1 stage with a blade tip linear velocity of 20m / s to 50m / s and a time of 10min to 30min; A2 stage with a blade tip linear velocity of 40m / s to 60m / s and a time of 3min to 30min; and A3 stage with a blade tip linear velocity of 10m / s to 30m / s and a time of 10min to 30min. And / or, In step S2, the second mixing is carried out in a paddle mixer, which includes the following stages performed sequentially: stage B1 with a blade tip linear velocity of 10m / s to 20m / s and a time of 3min to 10min, and stage B2 with a blade tip linear velocity of 30m / s to 50m / s and a time of 3min to 10min.

4. The method for preparing the dry cathode material according to claim 3, characterized in that, During the first mixing process, the A1 stage is carried out at 25±2℃, the A2 stage is carried out at 100℃~150℃, and the A3 stage is carried out at 20℃~50℃. And / or, During the second mixing process, the B1 stage is carried out at 5°C to 15°C, and the B2 stage is carried out at 80°C to 100°C.

5. The method for preparing the dry cathode material according to any one of claims 1 to 4, characterized in that, The weight ratio of the first adhesive to the second adhesive is 1:(1~3); and / or, The weight ratio of the first conductive agent to the second conductive agent is (0.3~0.7):(0.5~1.2); and / or, Based on the weight of the dry-process cathode material as 100%, the content of the cathode active material is 96.5%~98.5%, the total content of the first conductive agent and the second conductive agent is 0.5%~1.5%, and the total content of the first binder and the second binder is 1.0%~2.0%.

6. A dry-process cathode material, characterized in that, The dry cathode material is prepared by the method for preparing dry cathode material according to any one of claims 1 to 5.

7. A dry-process positive electrode sheet, comprising a positive electrode active material, characterized in that, The positive electrode active material includes the dry-process positive electrode material as described in claim 6.

8. A method for preparing a dry-process positive electrode, characterized in that, include: Step R1: Prepare a self-supporting membrane from the dry cathode material described in claim 6; Step R2: Combine the self-supporting membrane with the current collector to form the dry-process positive electrode.

9. The method for preparing a dry-process positive electrode sheet according to claim 8, characterized in that, Step R1 includes: preparing the self-supporting membrane from the dry cathode material by multi-stage differential rolling at 80℃~120℃, with a rolling pressure of 200kg / cm~800kg / cm and a speed ratio greater than 1. Step R2 includes: at 100℃~140℃, with a rolling pressure of 300kg / cm~1000kg / cm, combining the self-supporting membrane and the current collector to form the dry-process positive electrode sheet; Preferably, the self-supporting membrane has a thickness of 90 μm to 200 μm, a tensile strength of 0.1 MPa to 1 MPa, and an electrode resistivity of 30 Ω. cm~90Ω cm.

10. A secondary battery, comprising a positive electrode, characterized in that, The positive electrode is the dry-process positive electrode as described in claim 7; or, the positive electrode is prepared by the method for preparing the dry-process positive electrode as described in claim 8 or 9.

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