Battery cell and method for producing the same, battery device, electric device, energy storage device

Abstract

This invention provides a battery cell, its preparation method, a battery device, an electrical device, and an energy storage device. The method for preparing the positive electrode in the battery cell includes: firstly mixing carbon-coated lithium iron phosphate material, a first binder, and a conductive agent to obtain a first mixture; secondly mixing the first mixture and a second binder to form a fiber to obtain a second mixture; rolling the second mixture into a film to obtain a self-supporting electrode film; and composite the self-supporting electrode film with a current collector to obtain the positive electrode. The second binder includes polytetrafluoroethylene (PTFE); the first binder is polyvinylidene fluoride (PVDF) and / or PVDF-hexafluoropropylene copolymer. The positive electrode prepared by the above method possesses good self-supporting film integrity, tensile strength, interfacial adhesion, and electrochemical performance, making it suitable for high-capacity, long-cycle-life lithium iron phosphate battery cells and related devices.

Description

Technical Field

[0001] This application relates to the field of batteries, and in particular to a battery cell and its preparation method, a battery device, an electrical device, and an energy storage device. Background Technology

[0002] Lithium iron phosphate (LiFePO4, LFP) batteries have become the mainstream technology in the energy storage field due to their excellent cycle stability, outstanding safety performance, and continuously decreasing manufacturing costs. Currently, energy storage cells are developing towards larger capacities, which places higher demands on the areal density and thickness of the positive electrode.

[0003] Dry process electrode (DPE) technology, as a solvent-free electrode fabrication method, applies mechanical shear force to electrode powder containing fibrous binders such as polytetrafluoroethylene (PTFE), causing the PTFE to fibroinate in situ to form a three-dimensional network framework. This framework then bonds active materials and conductive agents into a self-supporting electrode film. Compared to traditional wet coating processes, dry process electrode technology eliminates the need for toxic organic solvents such as N-methylpyrrolidone (NMP) and large-scale drying equipment. It offers significant advantages in reducing manufacturing costs, shortening production cycles, and minimizing environmental pollution, and is considered an important technological direction for next-generation lithium-ion battery electrode manufacturing.

[0004] Currently, only dry preparation processes based on the PTFE-PVDF binary binder system have been developed for high-nickel ternary cathode materials. However, a systematic dry preparation technology for lithium iron phosphate (LFP) cathode materials remains lacking. LFP and high-nickel ternary materials differ fundamentally in crystal structure, particle morphology, mechanical properties, and surface chemistry. These differences pose significant challenges to directly applying the process parameters and material ratios of high-nickel ternary dry electrode preparation to LFP dry electrode fabrication. Therefore, it is urgent to develop a specialized dry electrode preparation technology tailored to the inherent characteristics of LFP materials to meet the pressing demand of the energy storage market for high-quality cathode electrodes for large-capacity LFP cells. Summary of the Invention

[0005] The main objective of this invention is to provide a battery cell and its preparation method, battery device, power consumption device, and energy storage device, in order to solve the problem of over-fiberization in the preparation of lithium iron phosphate dry electrodes in existing battery cells.

[0006] To achieve the above objectives, according to one aspect of the present invention, a method for preparing a battery cell is provided, comprising preparing a positive electrode sheet by sequentially stacking and assembling a positive electrode sheet, a separator, and a negative electrode sheet to obtain a battery cell. The method for preparing the positive electrode sheet includes: step S1, performing a first mixing of raw materials including carbon-coated lithium iron phosphate material, a first binder, and a conductive agent to obtain a first mixture; the equivalent shear rate of the first mixture is <10s. -1 Step S2: The raw materials including the first mixture and the second binder are fiberized by a second mixing process to obtain the second mixture; the equivalent shear rate of the second mixture is greater than that of the first mixture, and the specific mixing energy of the second mixture is 0.01~0.5kWh / kg; Step S3: The second mixture is rolled into a film to obtain a self-supporting electrode film; Step S4: The self-supporting electrode film and the current collector are composited to obtain a positive electrode sheet; wherein, the second binder includes polytetrafluoroethylene; the first binder is polyvinylidene fluoride and / or polyvinylidene fluoride-hexafluoropropylene copolymer.

[0007] Furthermore, in step S1, the temperature of the first mixing is 25~50℃, the time of the first mixing is 5~30min, and the equivalent shear rate of the first mixing is 1~9s. -1 .

[0008] Further, in step S1, the mass ratio of the carbon-coated lithium iron phosphate material, the first binder, and the conductive agent is 86~98:0.5~6:1~8; and / or, the D50 particle size of the primary particles of the carbon-coated lithium iron phosphate material is 0.5~5μm, and the BET specific surface area of ​​the carbon-coated lithium iron phosphate material is 8~25m². 2 The tap density of carbon-coated lithium iron phosphate material is 0.8~1.5 g / cm³. 3 The carbon coating layer on the surface of the carbon-coated lithium iron phosphate material has a mass content of 0.5~3wt%.

[0009] Further, in step S1, the D50 particle size of the first binder is 1~50μm, the weight-average molecular weight of the first binder is 200000~800000g / mol; and / or, the conductive agent is selected from any one or more of carbon black, carbon nanotubes, carbon fibers, and graphene; the aspect ratio of the carbon nanotubes is ≥50.

[0010] Furthermore, in step S2, the equivalent shear rate of the second mixture is 10~200 s. -1 The temperature of the second mixing is 25~90℃, the specific mixing energy of the second mixing is 0.02~0.2kWh / kg, and the time of the second mixing is 5~60min.

[0011] Further, in step S2, the mass ratio of the second binder to the first binder is (1:4) to (4:1); and / or, the D50 particle size of the second binder is 100~500μm, and the density of the second binder is 2.0~2.3g / cm³. 3 .

[0012] Further, in step S2, the second binder is a modified second binder with inorganic nanoparticles loaded on its surface. The inorganic nanoparticles are silicon dioxide and / or aluminum oxide, and the loading amount of the inorganic nanoparticles is 0.05~0.5wt%.

[0013] Further, in step S2, the second binder in the second mixture is added in stages and multiple times, with each addition amounting to 20-40% of the total mass of the second binder, and the interval between two adjacent additions being 2-10 minutes; and / or, in step S2, the second mixing process includes a third mixing and a fourth mixing performed sequentially; the equivalent shear rate of the third mixing is 10-80 s. -1 The temperature of the third mixing is 25~70℃, the specific mixing energy of the third mixing is 0.01~0.1kWh / kg, and the time of the third mixing accounts for 20~50% of the total time of the second mixing; the equivalent shear rate of the fourth mixing is 10~120s higher than that of the third mixing. -1 The temperature of the fourth mixture is 5-30°C higher than that of the third mixture, and the specific mixing energy of the fourth mixture is 0.01-0.15 kWh / kg higher than that of the third mixture.

[0014] Furthermore, the temperature for roll forming is 25~120℃, the roll forming line pressure is 5~50MPa, and the number of roll forming passes is 1~5; and / or, the lamination method is hot pressing, the hot pressing temperature is 100~200℃, the hot pressing pressure is 1~20MPa, and the hot pressing time is 1~30s; and / or, the current collector is aluminum foil and / or carbon-coated aluminum foil.

[0015] According to another aspect of the present invention, a battery cell is provided, comprising a positive electrode, a separator, and a negative electrode, wherein the positive electrode comprises a current collector and a self-supporting electrode film, and the battery cell is prepared by the preparation method described above.

[0016] Furthermore, the self-supporting electrode film includes carbon-coated lithium iron phosphate material, a second binder, a first binder, and a conductive agent; wherein the mass of the second binder accounts for 0.5 to 5% of the total mass of the self-supporting electrode film; the mass of the first binder accounts for 0.5 to 6% of the total mass of the self-supporting electrode film; and the mass of the conductive agent accounts for 1 to 8% of the total mass of the self-supporting electrode film.

[0017] Furthermore, the second binder forms a three-dimensional continuous fiber network structure in the self-supporting electrode film, wherein the average length of the fibers in the three-dimensional continuous fiber network is 10~200μm, and the fiber density of the three-dimensional continuous fiber network is 1~20 fibers / μm. 2 ; and / or, the thickness of the current collector is 10~20μm; and / or, the thickness of the self-supporting electrode film is 100~500μm, the tensile strength of the self-supporting electrode film is 3~30MPa, the porosity of the self-supporting electrode film is 20~50%, and the areal density of the self-supporting electrode film is 15~50mg / cm³. 2 .

[0018] According to another aspect of the present invention, a battery device is provided, the battery device comprising the above-described battery cell, and the battery device comprising any one or more of a battery module, a battery pack, and an energy storage battery.

[0019] According to another aspect of the present invention, an electrical device is provided, which includes the battery device described above, the battery device being used to provide electrical energy.

[0020] According to another aspect of the present invention, an energy storage device is provided, which includes the battery device described above, the battery device being used to store electrical energy.

[0021] The technical solution of this application has the following beneficial effects: Addressing the problem of high hardness, small particle size, and strong surface stress transmission in carbon-coated lithium iron phosphate particles, which easily leads to over-fiberization of polytetrafluoroethylene (PTFE), this application employs a two-step process of "low-shear pre-dispersion + controlled fiberization mixing," and constructs a synergistic system consisting of a first binder buffer filling, a second binder fiberization skeleton, and a conductive agent for assisted guidance. In the first mixing stage, the first binder and conductive agent are pre-dispersed on the surface and interparticle spaces of the lithium iron phosphate particles, reducing the local shear impact of subsequent hard particles on the second binder fibers. In the second mixing stage, under limited shear rate and specific mixing energy, the second binder is moderately fiberized, forming a continuous but not excessively tensile three-dimensional network. After roll forming and composite with a current collector, the resulting positive electrode sheet possesses good self-supporting film integrity, tensile strength, interfacial adhesion, and electrochemical performance, making it suitable for high-capacity, long-cycle-life lithium iron phosphate battery cells and related devices. Attached Figure Description

[0022] 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:

[0023] Figure 1 A schematic flowchart of the method for preparing a single battery cell in Embodiment 1 of this application is shown. Detailed Implementation

[0024] 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 accompanying drawings and embodiments.

[0025] As analyzed in the background section of this application, the preparation of lithium iron phosphate dry electrodes in existing battery cells suffers from over-fiberization. To address this issue, this application provides a battery cell, its preparation method, a battery device, an electrical device, and an energy storage device.

[0026] Dry electrode technology developed based on high-nickel ternary materials faces fundamental technical obstacles when directly applied to lithium iron phosphate (LFP) cathode systems. The core issue lies in the reversal of the fibrous behavior caused by the difference in physical properties between LFP and NMC / NCA materials. Specifically, high-nickel ternary materials (such as NMC811) have a Mohs hardness of approximately 3-4, and their primary particles typically aggregate to form secondary spherical particles with a diameter of 10-15 μm, lacking a carbon coating. During dry mixing, the softer NMC secondary particles are prone to partial breakage under shear force, absorbing a significant proportion of the shear energy. This results in a relatively mild effective shear force transferred to the PTFE particles, making the fibrous process easier to control. Therefore, the core challenge of NMC dry electrodes lies in providing sufficient shear energy to achieve "full fibrousization" of PTFE, essentially a problem of "under-fibrousization."

[0027] In contrast, the olivine structure of lithium iron phosphate (LFP) materials endows them with a Mohs hardness of 5–6 (significantly higher than NMC), and their primary particle size is typically only 0.5–2 μm (much smaller than NMC secondary particles). Furthermore, industrially produced LFP materials generally undergo carbon coating treatment (surface carbon content of 0.5–3 wt%). The synergistic effect of these three physical properties has a drastically different impact on the PTFE fiberization process: high hardness means that LFP particles are less prone to breakage during shearing, unlike NMC which absorbs shear energy through deformation; instead, they almost completely transfer mechanical stress to the PTFE fibers. The small particle size results in a much higher specific surface area per unit mass of LFP (typically 3–5 times that of NMC), leading to a thinner PVDF dispersion in the interparticle spaces, reducing the protective ability of the fibers, and simultaneously creating a dense "cutting" effect on the PTFE fibers from numerous tiny, hard particles. The high coefficient of friction of the carbon coating further enhances the stress transfer efficiency between the particles and PTFE. The combined effect of these three factors is that, under the same dry mixing process conditions, the PTFE fibers in the LFP system experience tensile stress far exceeding that of the NMC system. This leads to excessive fiber stretching and breakage, disrupting the continuity of the three-dimensional network. Ultimately, this manifests as embrittlement of the electrode film, insufficient strength, cracking or powder shedding during rolling, and the inability to form a self-supporting film with industrial usability. This is the "overfiberization" problem defined in this invention.

[0028] Currently, some studies introducing PVDF are mainly based on electrochemical motivation (compensating for the electrochemical instability of PTFE at low potentials), without addressing the aforementioned problem of uncontrolled fibrosis caused by differences in the hardness of active materials. Their process parameters (shear strength, mixing time, number of rolling passes, etc.) are optimized around the goal of "overcoming the shear energy absorption of NMC soft particles to achieve sufficient fibrosis." Directly applying these parameters to LFP systems not only fails to solve the over-fiberization problem but also exacerbates it, thus hindering the application and promotion of dry electrode technology in the LFP cathode field.

[0029] In a typical embodiment of this application, a method for preparing a battery cell is provided, including preparing a positive electrode sheet by sequentially stacking and assembling a positive electrode sheet, a separator, and a negative electrode sheet to obtain a battery cell. The method for preparing the positive electrode sheet includes: step S1, performing a first mixing of raw materials including carbon-coated lithium iron phosphate material, a first binder, and a conductive agent to obtain a first mixture; the equivalent shear rate of the first mixture is <10s. -1 Step S2: The raw materials including the first mixture and the second binder are fiberized by a second mixing process to obtain the second mixture; the equivalent shear rate of the second mixture is greater than that of the first mixture, and the specific mixing energy of the second mixture is 0.01~0.5kWh / kg; Step S3: The second mixture is rolled into a film to obtain a self-supporting electrode film; Step S4: The self-supporting electrode film and the current collector are composited to obtain a positive electrode sheet; wherein, the second binder includes polytetrafluoroethylene; the first binder is polyvinylidene fluoride and / or polyvinylidene fluoride-hexafluoropropylene copolymer.

[0030] Based on an in-depth mechanistic analysis of the "over-fiberization" problem in lithium iron phosphate materials, this application proposes a technical solution that contrasts with existing high-nickel ternary dry electrode technologies. The core design of high-nickel ternary dry electrode technology lies in "how to promote sufficient fiberization," while the core design of this application lies in "how to control the degree of fiberization and prevent excessive fiberization." This shift in approach stems from a systematic study of the triple coupling effect of hardness, particle size, and surface properties in lithium iron phosphate materials. Therefore, this application constructs a ternary synergistic system guided by a second binder (fiberized binder) fiberized skeleton, a first binder (non-fiberized binder) filling buffer, and a conductive agent. The second binder undertakes the function of constructing the fiberized network skeleton, providing self-support for the electrode film. The first binder is dispersed in the interparticle gaps of the carbon-coated lithium iron phosphate material, acting as a "shear buffer layer" between hard particles and PTFE fibers during the mixing stage, thereby reducing local stress concentration. During the rolling stage, the first binder softens and forms a continuous point-bonded network, providing supplementary bonding force and repair capabilities for fiber breakage points. While constructing a conductive network, the conductive agent guides the displacement direction of lithium iron phosphate particles during the mixing process, promoting controlled directional fiber growth rather than random breakage. Specifically, in step S1, the raw materials of carbon-coated lithium iron phosphate material, the first binder, and the conductive agent are first mixed, and the equivalent shear rate of the first mixture is controlled within the aforementioned range. This allows these components to be premixed under low shear conditions, achieving uniform dispersion of each component. In particular, it ensures that the first binder is uniformly dispersed in the gaps between LFP particles, forming a preliminary "buffer layer" distribution. At the same time, the conductive agent particles or fibers must be uniformly wrapped on the surface of the LFP, and the fiberization of the second binder must be avoided at this stage. In step S2, the second binder is added separately to the first mixture, and fiberization mixing is performed using a relatively high shear force. The fiberization mixing energy required for the LFP system needs to be lower than that for the high-nickel ternary system because the hard LFP particles themselves provide efficient shear transfer for the fiberization process. Controlling the specific mixing energy of the second mixture within the aforementioned range can prevent the second binder from becoming over-fiberized. A self-supporting electrode film is formed by roll forming, thereby better combining with the current collector to form a positive electrode sheet. Therefore, the preparation method of this application can simultaneously optimize tensile strength, self-support and electrochemical activity under the harsh conditions of high hardness and small particle size of LFP materials, thereby improving the electrochemical performance of battery cells and better meeting the application scenarios of large-capacity and long-cycle-life batteries.

[0031] Therefore, the key to the above preparation method is not simply increasing the degree of fiberization of the second binder, but establishing a controlled fiberization window in the lithium iron phosphate system. Step S1 first pre-disperses the first binder and conductive agent at low shear rates, forming a buffer and conductive / directing base. Step S2 then adds the second binder and triggers polytetrafluoroethylene fiberization with a higher but controlled shear rate and specific mixing energy. As a result, the shear transmission generated by the hard lithium iron phosphate particles is buffered by the first binder, the second binder fibers can form a continuous network without being easily overstretched and broken, and the conductive agent simultaneously constructs conductive pathways and assists in fiber orientation. This synergistic effect enables the resulting self-supporting electrode film to maintain good mechanical integrity, current collector adhesion, and electrochemical stability even under thick film and high areal density conditions.

[0032] In one embodiment of this application, in step S1 above, the temperature of the first mixing is 25~50°C, the time of the first mixing is 5~30 min, and the equivalent shear rate of the first mixing is 1~9 s. -1 .

[0033] Preferably controlling the temperature, time, and equivalent shear rate of the first mixing within the aforementioned range helps to uniformly disperse the first binder particles into the gaps between the LFP particles, and the conductive agent particles or fibers must be uniformly coated on the LFP surface, thereby improving the uniformity of the self-supporting electrode film. Mixing uniformity can be determined through sampling particle size analysis and conductivity testing, wherein the coefficient of variation (CV) of particle size distribution does not exceed 15%. The first mixing can be performed using equipment such as a V-type mixer, a plow mixer, or a low-speed planetary mixer.

[0034] In one embodiment of this application, in step S1 above, the mass ratio of the carbon-coated lithium iron phosphate material, the first binder, and the conductive agent is 86~98:0.5~6:1~8; and / or, the D50 particle size of the primary particles of the carbon-coated lithium iron phosphate material is 0.5~5μm, and the BET specific surface area of ​​the carbon-coated lithium iron phosphate material is 8~25m². 2 The tap density of carbon-coated lithium iron phosphate material is 0.8~1.5 g / cm³. 3 The carbon coating layer on the surface of the carbon-coated lithium iron phosphate material has a mass content of 0.5~3wt%.

[0035] Preferably, the mass ratio of carbon-coated lithium iron phosphate material, the first binder, and the conductive agent within the aforementioned range helps to achieve efficient synergy between the binder and the conductive network while maintaining a high proportion of active materials, thereby improving the energy density and cycle stability of the cathode. The particle size and specific surface area of ​​LFP particles are the first-level parameters affecting the fibrous behavior during dry mixing; the smaller the particle size and the larger the specific surface area, the more significant the "cutting" effect on the second binder fiber network. Therefore, preferably, the D50 particle size, BET specific surface area, and tap density of the primary particles of the carbon-coated lithium iron phosphate material within the aforementioned range helps the LFP particles to stably transfer stress and disperse uniformly during mixing, thus facilitating the controllable construction of the second binder fiber network. Preferably, the mass content of the carbon coating layer on the surface of the carbon-coated lithium iron phosphate material within the aforementioned range helps to improve conductivity and reduce particle agglomeration, while maintaining stress coupling between hard particles and the second binder.

[0036] D50 particle size (also known as median particle size or 50% particle size) refers to the particle diameter that corresponds to the cumulative volume distribution reaching 50% in the particle size distribution.

[0037] In one embodiment of this application, in step S1 above, the D50 particle size of the first binder is 1~50μm, the weight-average molecular weight of the first binder is 200000~800000g / mol; and / or, the conductive agent is selected from any one or more of carbon black, carbon nanotubes, carbon fibers, and graphene; the aspect ratio of the carbon nanotubes is ≥50.

[0038] Preferred control of the D50 particle size and weight-average molecular weight of the first binder within the aforementioned range helps it serve as a shear buffer material between rigid LFP particles and PTFE fibers. The micron-sized solid particles of the first binder are dispersed in the gaps between the LFP particles, thereby alleviating overfiberization by reducing the efficiency of local shear stress transfer to the fibers during the mixing stage. At the same time, during the rolling stage, the first binder softens after the temperature rises to form a continuous point bonding network, providing supplementary bonding force at fiber breakage points, while also enhancing the adhesion between the self-supporting electrode film and the current collector.

[0039] The preferred conductive agent is a combination of carbon nanotubes and carbon black, with the mass content of carbon nanotubes being 0.2-1% and the mass content of carbon black being 1-5%. Carbon nanotubes with an aspect ratio within the above range not only facilitate the construction of a conductive network during dry mixing but also guide the displacement direction of LFP particles to a certain extent, thereby promoting the directional growth of PTFE fibers and contributing to improving the continuity and uniformity of the fiber network.

[0040] In one embodiment of this application, in step S2 above, the equivalent shear rate of the second mixture is 10~200s. -1The temperature of the second mixing is 25~90℃, the specific mixing energy of the second mixing is 0.02~0.2kWh / kg, and the time of the second mixing is 5~60min.

[0041] The fiberization mixing energy required for the LFP system is lower than that for the high-nickel ternary system. Since the hard LFP particles themselves provide efficient shear transfer for the fiberization process, controlling the equivalent shear rate, temperature, and specific mixing energy of the second mixing process within these ranges helps achieve "moderate fiberization" in the LFP system while avoiding "over-fiberization." This ensures the formation of a continuous three-dimensional network by the second binder while reducing structural damage caused by energy accumulation. Combined with the buffering effect of the first binder, this improves mechanical strength and electrode integrity. During the mixing process, the appearance and flowability of the material should be monitored: when the material gradually transforms from a loose powder into agglomerates with a certain degree of cohesion, it indicates that the fiberization of the second binder has reached an appropriate level. If the material continues to harden and shows signs of embrittlement, it indicates that it has entered an over-fiberization state, and mixing must be stopped immediately. Suitable equipment for the second mixing includes a high-speed mixer (stirring speed 500~3000 rpm), a twin-screw extruder (screw speed 50~300 rpm), an internal mixer (rotor speed 20~100 rpm), or an open mill (roller ratio 1:1.1~1.5). For twin-screw extruders and internal mixers, the mixing temperature can be precisely adjusted through the equipment's own temperature control system.

[0042] In one embodiment of this application, in step S2 above, the mass ratio of the second adhesive to the first adhesive is (1:4) to (4:1), preferably 0.8 to 2.5:1, more preferably 1 to 2:1; and / or, the D50 particle size of the second adhesive is 100 to 500 μm, and the density of the second adhesive is 2.0 to 2.3 g / cm³. 3 .

[0043] Preferably controlling the mass ratio of the second binder to the first binder within the aforementioned range helps to more accurately control the degree of fiberization. Simultaneously, this ratio can be determined in conjunction with the LFP particle characteristics and process conditions. When the LFP particle size is small and the specific surface area is large, the proportion of the first binder should be appropriately increased to enhance the buffering effect; conversely, it can be appropriately decreased. Preferably controlling the D50 particle size and density of the second binder within the aforementioned range helps to achieve stable, efficient, and controllable in-situ fiberization in the second mixing, thereby forming a uniform three-dimensional skeleton and improving the tensile strength and structural stability of the self-supporting membrane.

[0044] In one embodiment of this application, in step S2 above, the second binder is a modified second binder with inorganic nanoparticles loaded on its surface. The inorganic nanoparticles are silicon dioxide and / or aluminum oxide, and the loading amount of the inorganic nanoparticles is 0.05~0.5wt%.

[0045] Preferably, the second binder is a modified second binder with the aforementioned types of inorganic nanoparticles loaded on its surface. This helps to regulate fiberization kinetics and enhance the stability of the electrode structure during the second mixing process. The inorganic nanoparticles help to locally weaken the concentrated shear impact of the LFP hard particles on the PTFE fibers, alleviating over-fiberization fracture. Simultaneously, their high hardness and surface polarity can promote the anchoring and directional extension of the PTFE fibers in the interparticle gaps, thereby improving the continuity of the three-dimensional network. Furthermore, the inorganic nanoparticles help reduce the agglomeration of the second binder during the high-temperature rolling stage, thereby enhancing the compactness of the self-supporting electrode film. Optionally, the particle size of the inorganic nanoparticles is 10~200 nm, and the silica can be fluorinated silica.

[0046] In one embodiment of this application, in step S2 above, the second binder in the second mixture is added in stages and multiple times, with each addition amounting to 20-40% of the total mass of the second binder, and the interval between two adjacent additions being 2-10 minutes; and / or, in step S2, the second mixing process includes a third mixing and a fourth mixing performed sequentially; the equivalent shear rate of the third mixing is 10-80 s. -1 The temperature of the third mixing is 25~70℃, the specific mixing energy of the third mixing is 0.01~0.1kWh / kg, and the time of the third mixing accounts for 20~50% of the total time of the second mixing; the equivalent shear rate of the fourth mixing is 10~120s higher than that of the third mixing. -1 The temperature of the fourth mixture is 5-30°C higher than that of the third mixture, and the specific mixing energy of the fourth mixture is 0.01-0.15 kWh / kg higher than that of the third mixture.

[0047] The preferred method for adding the second binder in the second mixture is to add it in stages and multiple times, controlling the amount added in each stage and the interval between adjacent additions within the aforementioned range. This helps to achieve controllable fiberization and uniform network formation of the second binder. Staged addition helps to reduce the risk of excessively high local concentrations caused by a single addition, which can create "fibrillation hotspots" in high shear fields and lead to excessive stretching and breakage of local fibers. The aforementioned intervals allow sufficient time for the previously added second binder to form an initial fiber network skeleton on the surface of the LFP particles, providing a "guided growth substrate" for subsequent additions of the second binder. This promotes directional fiber extension and spatial uniform distribution, thereby improving the continuity and density of the fiber network.

[0048] The preferred second mixing process includes sequential third and fourth mixing. Controlling the equivalent shear rate, temperature, specific mixing energy, and time of the third and fourth mixing within the aforementioned ranges helps to achieve a two-stage regulation of "controllable nucleation-directional growth" in the fiberization of the second binder, thereby precisely matching the strong shear transfer characteristics brought about by the high hardness and small particle size of LFP. The third mixing uses low energy to gently initiate fiberization, forming a uniform and dense PTFE fiber network under the protection of the buffer layer. The fourth mixing increases the equivalent shear rate and temperature, inducing longitudinal fiber extension and cross-entanglement, constructing a highly connected, low-defect three-dimensional skeleton, thereby further improving the consistency of the intramembrane structure.

[0049] In one embodiment of this application, the temperature of roll forming is 25~120℃, the roll forming line pressure is 5~50MPa, and the number of roll forming passes is 1~5; and / or, the lamination method is hot pressing, the hot pressing temperature is 100~200℃, the hot pressing pressure is 1~20MPa, and the hot pressing time is 1~30s; and / or, the current collector is aluminum foil and / or carbon-coated aluminum foil.

[0050] Preferably controlling the roll forming temperature, roll forming line pressure, and roll forming passes within the aforementioned range helps to activate the thermal softening behavior of the first binder while maintaining the integrity of the PTFE fiber skeleton, achieving synergistic optimization of "structural strengthening" and "defect repair." The selection of the roll forming temperature must consider the softening behavior of the first binder; the temperature should be higher than the glass transition temperature of the first binder to ensure it has sufficient fluidity to fill fiber fracture points, while avoiding excessively high temperatures to reduce the agglomeration of the second binder particles at high temperatures.

[0051] Preferred control of the composite method, hot pressing temperature, pressure, and time within the above range helps to achieve efficient, uniform, and strong interfacial bonding between the self-supporting electrode membrane and the above-mentioned current collectors without damaging the PTFE fiber skeleton.

[0052] In another typical embodiment of this application, a battery cell is provided, including a positive electrode, a separator, and a negative electrode. The positive electrode includes a current collector and a self-supporting electrode film. The battery cell is prepared by the preparation method described above.

[0053] The positive electrode sheet prepared by the above method has advantages such as solvent-free, high mechanical strength, thick electrode compatibility, high energy density, and excellent cycle stability, thus enabling the battery cell to better meet the requirements of large capacity and long cycle life.

[0054] In one embodiment of this application, the self-supporting electrode film includes carbon-coated lithium iron phosphate material, a second binder, a first binder, and a conductive agent; wherein the mass of the second binder accounts for 0.5 to 5% of the total mass of the self-supporting electrode film; the mass of the first binder accounts for 0.5 to 6% of the total mass of the self-supporting electrode film; and the mass of the conductive agent accounts for 1 to 8% of the total mass of the self-supporting electrode film.

[0055] The self-supporting electrode film comprises carbon-coated lithium iron phosphate, a second binder, a first binder, and a conductive agent in the aforementioned mass percentages. This facilitates the construction of a ternary synergistic bonding system consisting of a fibrous framework of the second binder, a buffer-filling effect of the first binder, and guidance by the conductive agent. By controlling the degree of fibrosis of the second binder, the mechanical strength and electrochemical performance of the self-supporting electrode film are improved. Preferably, the total mass of the first and second binders accounts for 2-10% of the total mass of the self-supporting electrode film, more preferably 3-7%.

[0056] In one embodiment of this application, the second adhesive forms a three-dimensional continuous fiber network structure in the self-supporting electrode film, wherein the average length of the fibers in the three-dimensional continuous fiber network is 10~200μm, and the fiber density of the three-dimensional continuous fiber network is 1~20 fibers / μm. 2 ; and / or, the thickness of the current collector is 10~20μm; and / or, the thickness of the self-supporting electrode film is 100~500μm, the tensile strength of the self-supporting electrode film is 3~30MPa, the porosity of the self-supporting electrode film is 20~50%, preferably 25~45%, more preferably 30~40%, and the areal density of the self-supporting electrode film is 15~50mg / cm³. 2 .

[0057] The second binder forms a three-dimensional continuous fiber network structure in the self-supporting electrode film. Controlling the average length and fiber density of the fibers in the three-dimensional continuous fiber network within the aforementioned range helps to simultaneously optimize mechanical strength, structural stability, and ion transport efficiency. Optimizing the thickness of the current collector, and ensuring the thickness, tensile strength, porosity, and areal density of the self-supporting electrode film are within the aforementioned ranges, helps to further improve the volumetric energy density while maintaining the mechanical strength and conductivity of the positive electrode. High-nickel ternary dry electrodes typically employ a porosity of 22-35%. Considering the low ionic conductivity of LFP materials, increasing the porosity to 30-40% helps to provide more abundant electrolyte wetting channels, thereby compensating for the polarization loss caused by the increased ion transport path in the thick-film electrode, and thus improving the electrochemical performance of the thick-film LFP dry electrode.

[0058] The negative electrode sheet comprises a negative electrode current collector and a negative electrode active layer. The negative electrode active layer is formed by coating a negative electrode slurry onto the surface of the negative electrode current collector. The negative electrode slurry comprises a negative electrode active material, a conductive agent, and a binder. The negative electrode active material is selected from natural graphite, artificial graphite, soft carbon, hard carbon, silicon-carbon composites, and SiO₂.X Any one or more of the materials. Preferably, for energy storage applications, the negative electrode is made of artificial graphite, the mass content of the negative electrode active material is 93~97wt%, the mass content of the conductive agent is 0.5~3wt%, and the binder is a sodium carboxymethyl cellulose (CMC) / styrene-butadiene rubber (SBR) composite system with a mass content of 2~5wt%, which is coated on the copper foil current collector.

[0059] The separator is selected from any one of polyolefin separators, ceramic-coated separators, or composite functional separators. Specifically, the substrate can be selected from any one or more of polypropylene (PP) single-layer membranes, polyethylene (PE) single-layer membranes, or PP / PE / PP three-layer composite membranes, with a thickness of 7~25μm. Preferably, the surface of the separator is provided with a ceramic coating such as alumina, boehmite (AlOOH), or silica to improve thermal stability and electrolyte wettability.

[0060] The electrolyte contains lithium salts and organic solvents. The lithium salt is selected from one or more of lithium hexafluorophosphate (LiPF6), lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), lithium bis(fluorosulfonyl)imide (LiFSI), and lithium tetrafluoroborate (LiBF4), with a molar concentration of 0.8–1.5 mol / L. The organic solvent is selected from two or more of ethylene carbonate (EC), propylene carbonate (PC), dimethyl carbonate (DMC), diethyl carbonate (DEC), and methyl ethyl carbonate (EMC). The electrolyte may also contain functional additives, selected from vinylene carbonate (VC), fluoroethylene carbonate (FEC), propanesulfonate lactone (PS), etc.

[0061] In another typical embodiment of this application, a battery device is provided, which includes the above-mentioned battery cell, and the battery device includes any one or more of battery modules, battery packs, and energy storage batteries.

[0062] In another typical embodiment of this application, an electrical device is provided, which includes the battery device described above, the battery device being used to provide electrical energy.

[0063] Battery devices including the aforementioned battery cells have superior overall performance in terms of high power output, wide temperature range operation, and long cycle life, making them suitable for energy storage systems and power battery applications with high requirements for energy density, safety, and reliability.

[0064] The aforementioned battery devices enable electrical devices to have higher energy efficiency, longer service life and more stable power output characteristics, making them suitable for energy storage systems and electric vehicle applications with stringent requirements for safety and cycle performance.

[0065] In another typical embodiment of this application, an energy storage device is provided, which includes the battery device described above, the battery device being used to store electrical energy.

[0066] Energy storage devices including the aforementioned battery devices exhibit comprehensive advantages in terms of self-supporting film quality, mechanical strength, rate performance, and cycle stability, making them suitable for large-capacity energy storage system applications that have comprehensive requirements for energy density, cycle life, and safety.

[0067] In addition, the battery cells prepared in this application maintain a high initial discharge specific capacity while also having a high rate capacity retention rate and cycle capacity retention rate. They are suitable for energy storage battery and battery device applications with large capacity and long cycle life, including but not limited to 4-hour (4h) and 8-hour (8h) level energy storage power stations, grid peak shaving, and renewable energy smoothing output.

[0068] The beneficial effects of this application will be further illustrated below with reference to the embodiments.

[0069] Example 1

[0070] According to such Figure 1 The method shown is used to prepare the positive electrode sheet using the cell preparation method described above:

[0071] Positive electrode active material: carbon-coated lithium iron phosphate (LiFePO4 / C) powder, with a primary particle size of 1.5 μm (D50) and a BET specific surface area of ​​14 m². 2 / g, the surface carbon coating layer has a mass content of 1.5wt%, and the tap density is 1.1g / cm³. 3 Before use, LFP powder should be dried in a vacuum drying oven at 110°C for 8 hours to ensure that the moisture content is below 200 ppm.

[0072] First binder: polyvinylidene fluoride dry powder, with a D50 particle size of 1μm, a weight-average molecular weight of 400,000g / mol, and a purity of ≥99%.

[0073] Second binder: Polytetrafluoroethylene (PTFE) dry powder, dispersion type, D50 particle size 300μm, density 2.1g / cm³ 3 Purity ≥ 99%.

[0074] Conductive agent: Carbon nanotubes are multi-walled carbon nanotube powder (CNTs) with a diameter of 8-15 nm, a length of 1-5 μm, an aspect ratio of 50-1500, and a purity of not less than 95%. Conductive carbon black Super P has a primary particle size of 40 nm and a BET specific surface area of ​​62 m². 2 / g. The conductive agent is vacuum dried at 80°C for 4 hours before use.

[0075] The positive current collector is made of aluminum foil with a thickness of 15μm and the surface is degreased.

[0076] Preparation of the positive electrode: Carbon-coated lithium iron phosphate material, PVDF dry powder as the first binder, CNT conductive agent, and Super P conductive agent were added to a low-shear mixing device at a mass ratio of 92:2.5:0.5:2.5 and mixed at 35°C for 15 minutes to obtain a first mixture. The equivalent shear rate of the first mixture was 5 s⁻¹. -1 .

[0077] A second binder, PTFE dry powder (2.5% of the total mass of the self-supporting electrode film, with a mass ratio of 1:1), was added to the first mixture. The mixture was then transferred to a high-shear mixer and mixed at 60°C for 15 minutes to achieve fiberization, yielding the second mixture. The equivalent shear rate of the second mixture was 50 s⁻¹. -1 The specific mixing energy of the second mixture is 0.06 kWh / kg.

[0078] The second mixture was rolled into a film to obtain a self-supporting electrode film. The rolling temperature was 80℃, the rolling pressure was 20MPa, and the rolling process consisted of 3 passes.

[0079] A self-supporting electrode film and an aluminum foil current collector are composited by hot pressing to obtain a positive electrode sheet. The hot pressing temperature is 150℃, the pressure is 5MPa, and the hot pressing time is 10s. A second binder forms a three-dimensional continuous fiber network structure in the self-supporting electrode film.

[0080] Example 2

[0081] The difference from Example 1 is that the amount of the second binder PTFE dry powder added is 2% of the total mass of the self-supporting electrode film, and the mass ratio of the second binder to the first binder is 2:3, finally obtaining the positive electrode sheet.

[0082] Example 3

[0083] The difference from Example 1 is that the amount of the second binder PTFE dry powder added is 3% of the total mass of the self-supporting electrode film, and the mass ratio of the second binder to the first binder is 3:2, finally obtaining the positive electrode sheet.

[0084] Example 4

[0085] The difference from Example 1 is that the amount of the second binder PTFE dry powder added is 2% of the total mass of the self-supporting electrode film, and the mass ratio of the second binder to the first binder is 1:1, finally obtaining the positive electrode sheet.

[0086] Example 5

[0087] The difference from Example 1 is that the amount of the second binder PTFE dry powder added is 3% of the total mass of the self-supporting electrode film, and the mass ratio of the second binder to the first binder is 1:1, finally obtaining the positive electrode sheet.

[0088] Example 6

[0089] The difference from Example 1 is that the equivalent shear rate of the second mixture is 30 s. -1 The specific mixing energy of the second mixture is 0.03 kWh / kg, and the positive electrode is finally obtained.

[0090] Example 7

[0091] The difference from Example 1 is that the equivalent shear rate of the second mixture is 80 s. -1 The mixing energy of the second mixture is 0.1 kWh / kg, and the positive electrode is finally obtained.

[0092] Example 8

[0093] The difference from Example 1 is that the rolling temperature for film formation is 60°C, resulting in a positive electrode sheet.

[0094] Example 9

[0095] The difference from Example 1 is that the rolling temperature for film formation is 110°C, resulting in a positive electrode sheet.

[0096] Example 10

[0097] The difference from Example 1 is that the positive electrode active material is carbon-coated lithium iron phosphate (LiFePO4 / C) powder with a primary particle size of 0.8 μm and a BET specific surface area of ​​20 m². 2 / g, the surface carbon coating layer has a mass content of 2wt%. The amount of the second binder PTFE dry powder added is 2% of the total mass of the self-supporting electrode film, the mass ratio of the second binder to the first binder is 2:3, and the equivalent shear rate of the second mixture is 35s. -1 The specific mixing energy of the second mixture is 0.04 kWh / kg, and the positive electrode is finally obtained.

[0098] Example 11

[0099] The difference from Example 1 is that the positive electrode active material is carbon-coated lithium iron phosphate (LiFePO4 / C) powder with a primary particle size of 3 μm and a BET specific surface area of ​​10 m². 2 / g, the surface carbon coating layer has a mass content of 1wt%. The amount of the second binder PTFE dry powder added is 3% of the total mass of the self-supporting electrode film, the mass ratio of the second binder to the first binder is 3:2, and the equivalent shear rate of the second mixture is 60s. -1The specific mixing energy of the second mixture is 0.08 kWh / kg, and the positive electrode is finally obtained.

[0100] Example 12

[0101] The difference from Example 1 is that the porosity of the self-supporting electrode film is 30%, resulting in a positive electrode sheet.

[0102] Example 13

[0103] The difference from Example 1 is that the porosity of the self-supporting electrode film is 40%, resulting in a positive electrode sheet.

[0104] Example 14

[0105] The difference from Example 1 is that the thickness of the self-supporting electrode film is 170 μm and the areal density is 22 mg / cm³. 2 Finally, the positive electrode sheet is obtained.

[0106] Example 15

[0107] The difference from Example 1 is that the thickness of the self-supporting electrode film is 260 μm and the areal density is 30 mg / cm³. 2 Finally, the positive electrode sheet is obtained.

[0108] Example 16

[0109] The difference from Example 1 is that the amount of the second binder PTFE dry powder added is 2% of the total mass of the self-supporting electrode film, the mass ratio of the second binder to the first binder is 2:2.5, and the equivalent shear rate of the second mixture is 45s. -1 The mixing energy of the second mixture is 0.05 kWh / kg, the rolling temperature is 90℃, and finally the positive electrode sheet is obtained.

[0110] Example 17

[0111] The difference from Example 1 is that the second binder is a modified second binder PTFE dry powder with inorganic nanoparticles loaded on its surface. The inorganic nanoparticles are silicon dioxide with a particle size of 50nm and the loading amount of inorganic nanoparticles is 0.2wt%, and finally a positive electrode sheet is obtained.

[0112] Example 18

[0113] The difference from Example 1 is that the second binder in the second mixture is added in stages and multiple times, four times, with each addition amounting to 25% of the total mass of the second binder, and the interval between two adjacent additions being 3 minutes, ultimately yielding a positive electrode sheet.

[0114] Example 19

[0115] The difference from Example 1 is that the second mixing process includes a third and a fourth mixing performed sequentially; the equivalent shear rate of the third mixing is 30 s. -1 The temperature was 40℃, the specific mixing energy was 0.03 kWh / kg, and the third mixing time accounted for 30% of the total second mixing time; the equivalent shear rate of the fourth mixing was 60 s. -1 The temperature was 60℃, the specific mixing energy was 0.05kWh / kg, and finally the positive electrode sheet was obtained.

[0116] Example 20

[0117] The difference from Example 1 is that the conductive agent is only carbon nanotubes, and the mass content of the conductive agent remains unchanged, ultimately resulting in a positive electrode sheet.

[0118] Example 21

[0119] The difference from Example 1 is that the porosity of the self-supporting electrode film is 22%, resulting in a positive electrode sheet.

[0120] Example 22

[0121] The difference from Example 1 is that the aspect ratio of the carbon nanotubes is 40, resulting in a positive electrode.

[0122] Example 23

[0123] The difference from Example 1 is that the positive electrode active material is carbon-coated lithium iron phosphate (LiFePO4 / C) powder with a primary particle size of 0.5 μm and a BET specific surface area of ​​8 m². 2 / g, the surface carbon coating layer has a mass content of 0.5wt%, and the tap density is 0.8g / cm³. 3 .

[0124] First binder: Polyvinylidene fluoride-hexafluoropropylene copolymer dry powder, with a D50 particle size of 20μm, a weight-average molecular weight of 200,000g / mol, and a purity of ≥99%.

[0125] Second binder: Polytetrafluoroethylene (PTFE) dry powder, dispersion type, D50 particle size 100μm, density 2.0g / cm³. 3 Purity ≥ 99%.

[0126] Conductive agent: Carbon nanotubes are multi-walled carbon nanotube powder (CNTs) with an aspect ratio of 50-500 and a purity of not less than 95%. Conductive carbon black Super P has a primary particle size of 30 nm and a BET specific surface area of ​​80 m². 2 / g.

[0127] The positive current collector is made of aluminum foil with a thickness of 10μm and the surface is degreased.

[0128] Preparation of the positive electrode: Carbon-coated lithium iron phosphate material, polyvinylidene fluoride-hexafluoropropylene copolymer dry powder as the first binder, CNT conductive agent, and Super P conductive agent were added to a low-shear mixing device at a mass ratio of 91.5:0.5:3.5:4.5 and mixed at 25°C for 30 minutes to obtain the first mixture. The equivalent shear rate of the first mixture was 3 s⁻¹. -1 .

[0129] A second binder, PTFE dry powder (1.5% of the total mass of the self-supporting electrode film, with a mass ratio of 1.5:0.5), was added to the first mixture. The mixture was then transferred to a high-shear mixer and mixed at 25°C for 30 minutes to achieve fiberization, yielding the second mixture. The equivalent shear rate of the second mixture was 10 s⁻¹. -1 The specific mixing energy of the second mixture is 0.01 kWh / kg.

[0130] The second mixture was rolled into a film to obtain a self-supporting electrode film. The rolling temperature was 25°C, the rolling pressure was 50 MPa, and the rolling process was performed in one pass.

[0131] A self-supporting electrode film and an aluminum foil current collector are composited by hot pressing to obtain a positive electrode sheet. The hot pressing temperature is 100℃, the pressure is 20MPa, and the hot pressing time is 30s. A second binder forms a three-dimensional continuous fiber network structure in the self-supporting electrode film.

[0132] Example 24

[0133] The difference from Example 1 is that the positive electrode active material is carbon-coated lithium iron phosphate (LiFePO4 / C) powder with a primary particle size of 5 μm and a BET specific surface area of ​​8 m². 2 / g, the surface carbon coating layer has a mass content of 3wt%, and the tap density is 1.5g / cm³. 3 .

[0134] First binder: Polyvinylidene fluoride-hexafluoropropylene copolymer dry powder, with a D50 particle size of 50μm, a weight-average molecular weight of 800,000g / mol, and a purity of ≥99%.

[0135] Second binder: Polytetrafluoroethylene (PTFE) dry powder, dispersion type, D50 particle size 500μm, density 2.3g / cm³. 3 Purity ≥ 99%.

[0136] Conductive agent: Carbon nanotubes are multi-walled carbon nanotube powder (CNTs) with an aspect ratio of 50~1500 and a purity of not less than 95%. Conductive carbon black Super P has a primary particle size of 50nm and a BET specific surface area of ​​100m².2 / g.

[0137] The positive current collector is made of aluminum foil with a thickness of 20μm and the surface is degreased.

[0138] Preparation of the positive electrode: Carbon-coated lithium iron phosphate material, polyvinylidene fluoride-hexafluoropropylene copolymer dry powder as the first binder, CNT conductive agent, and Super P conductive agent were added to a low-shear mixing device at a mass ratio of 93:6:0.5:0.5 and mixed at 50°C for 5 minutes to obtain a first mixture. The equivalent shear rate of the first mixture was 9 s. -1 .

[0139] A second binder, PTFE dry powder (5% of the total mass of the self-supporting electrode film, with a mass ratio of 5:6), was added to the first mixture. The mixture was then transferred to a high-shear mixer and mixed at 90°C for 5 minutes to achieve fiberization, yielding the second mixture. The equivalent shear rate of the second mixture was 200 s⁻¹. -1 The specific mixing energy of the second mixture is 0.5 kWh / kg.

[0140] The second mixture was rolled into a film to obtain a self-supporting electrode film. The rolling temperature was 120℃, the rolling pressure was 5MPa, and the rolling process consisted of 5 passes.

[0141] A self-supporting electrode film and an aluminum foil current collector are composited by hot pressing to obtain a positive electrode sheet. The hot pressing temperature is 200℃, the pressure is 1MPa, and the hot pressing time is 1s. A second binder forms a three-dimensional continuous fiber network structure in the self-supporting electrode film.

[0142] Example 25

[0143] The difference from Example 1 is that the amount of the second binder PTFE dry powder added is 1% of the total mass of the self-supporting electrode film, and the mass ratio of the second binder to the first binder is 5:1, finally obtaining the positive electrode sheet.

[0144] Example 26

[0145] The difference from Example 1 is that the rolling temperature for film formation is 20°C, resulting in a positive electrode sheet.

[0146] Comparative Example 1

[0147] The difference from Example 1 is that the carbon-coated lithium iron phosphate material, the first binder PVDF dry powder, the conductive agent CNT, the conductive agent Super P, and the second binder PTFE dry powder (the amount added is 4% of the total mass of the self-supporting electrode film, and the mass ratio of the first binder to the second binder is 4:1) are mixed in a high-shear mixing device at 60°C to achieve fibrosis, resulting in a second mixture. The equivalent shear rate of the second mixture is 220 s. -1 The specific mixing energy of the second mixture is 0.6 kWh / kg, and the positive electrode is finally obtained.

[0148] Comparative Example 2

[0149] The difference from Example 1 is that the first binder is not added, and the amount of the second binder added is 5% of the total mass of the self-supporting electrode film, and finally a positive electrode sheet is obtained.

[0150] Comparative Example 3

[0151] The difference from Example 1 is that no second binder is added; the amount of the first binder added is 5% of the total mass of the self-supporting electrode film. The first mixture is directly rolled into a film to obtain the self-supporting electrode film. The self-supporting electrode film and the current collector are then combined to obtain the positive electrode sheet.

[0152] Comparative Example 4

[0153] The difference from Example 1 is that the carbon-coated lithium iron phosphate material, the first binder PVDF dry powder, the conductive agent CNT, the conductive agent Super P, and the second binder PTFE dry powder are mixed in a high-shear mixing device to achieve fibrosis, thereby obtaining a second mixture and finally obtaining a positive electrode sheet.

[0154] Comparative Example 5

[0155] The difference from Example 1 is that the equivalent shear rate of the second mixture is 220 s. -1 The specific mixing energy of the second mixture is 0.6 kWh / kg, and the positive electrode is finally obtained.

[0156] Comparative Example 6

[0157] The difference from Example 1 is that the equivalent shear rate of the first mixture is 15s. -1 Finally, the positive electrode sheet is obtained.

[0158] Comparative Example 7

[0159] The difference from Example 1 is that carbon-coated lithium iron phosphate material, PVDF dry powder as the first binder, CNT conductive agent, and Super P conductive agent are added to the organic solvent N-methylpyrrolidone to obtain a positive electrode slurry. The positive electrode slurry is coated on the surface of the current collector and dried to obtain a positive electrode sheet.

[0160] Test method:

[0161] Electrochemical performance verification was conducted using 3-5Ah lithium iron phosphate / graphite stacked small pouch cells as a unified testing platform. For samples with defects in the film formation but still yielding locally intact electrode areas, the intact areas were selected for composite processing and battery assembly; samples that could not form a continuous self-supporting electrode film were exempted from subsequent assembly and electrochemical testing.

[0162] The negative electrode is a wet-coated artificial graphite electrode with a negative electrode active material content of 95 wt%, a CMC / SBR composite binder content of 3.5 wt%, and a Super P conductive agent content of 1.5 wt%, coated on an 8 μm thick copper foil current collector. The N / P ratio (negative electrode reversible capacity / positive electrode reversible capacity) is controlled within the range of 1.05 to 1.10. The separator is a 20 μm thick polypropylene / polyethylene / polypropylene (PP / PE / PP) three-layer composite separator with an alumina ceramic coating on the surface. The electrolyte uses lithium hexafluorophosphate as the conductive lithium salt with a molar concentration of 1.0 mol / L, dissolved in an organic solvent containing ethylene carbonate (EC), dimethyl carbonate (DMC), and ethyl methyl carbonate (EMC) in a volume ratio of 1:1:1, with 2 wt% vinylene carbonate (VC) added as a film-forming additive.

[0163] Battery assembly employs a standard stacking process, conducted in a dry room with a dew point below -40°C. The positive electrode, separator, and negative electrode are stacked sequentially and sealed in an aluminum-plastic film. Electrolyte injection is performed inside a glove box. After injection, the battery is allowed to stand for 12 hours to allow the electrolyte to fully wet the electrode pores. Formation employs a low-current stepped charging strategy: the first cycle is a constant current charge at 0.05C to 3.65V, followed by three cycles of 0.1C / 0.1C charge-discharge for activation, with a voltage window from 2.5V to 3.65V. After formation, the battery is left to stand at 25°C for 24 hours to stabilize the interface before subsequent performance testing.

[0164] To systematically evaluate the overall performance of the dry-process self-supported lithium iron phosphate cathode films prepared in each embodiment, comparative example, and reference example, performance tests were performed on all samples using the following uniform method. All film-level test data are based on the average of five parallel sample tests. All battery-level test data are based on the average of three parallel battery tests.

[0165] 1) Determination of self-supporting electrode film formation

[0166] After the roll forming process, the self-supporting electrode films were uniformly evaluated for appearance and operability. The evaluation included: whether the film could be removed intact from the roll press; whether there were visible cracks or powder shedding on the film surface; and whether the film could withstand manual bending (bending radius approximately 10 mm) without breaking. Based on the evaluation results, the film formation status was divided into four levels: intact film (smooth and uniform surface, no cracks or powder shedding, bendable without breaking), edge defects (intact central area but minor damage at the edges), severe cracking and powder shedding (large area cracks or significant powder detachment), and no film formation (material in a loose powder state, unable to form a continuous film). The self-supporting film formation determination is the most direct indicator for evaluating whether over-fibrillation is effectively controlled, and is used to classify and characterize the film formation quality of all samples. Five parallel samples were tested for each sample, and the average value was taken.

[0167] 2) Tensile strength

[0168] At 25℃, the self-supporting electrode film was cut into strips with a width of 10 mm and a gauge length of 50 mm. Uniaxial tensile tests were performed on a universal testing machine at a tensile rate of 5 mm / min. The maximum stress at fracture was recorded as the tensile strength (MPa). The tensile strength reflects whether the self-supporting electrode film meets the mechanical requirements of subsequent transfer, roll forming, and battery assembly processes. Five parallel samples were tested for each sample, and the average value was taken.

[0169] 3) 180° peel strength

[0170] After hot-pressing the self-supporting electrode film with the aluminum foil current collector, the samples were cut into strips with a width of 25 mm. A 180° peel test was performed on a universal testing machine at a peel rate of 50 mm / min. The average peel force during the stable peeling stage was recorded, and the peel strength (N / cm) was obtained by dividing the average peel force by the sample width. The peel strength reflects the effectiveness of the first adhesive in softening and forming a bonding transition layer during the hot-pressing stage, as well as the interfacial bonding quality between the self-supporting electrode film and the current collector. Five parallel samples were tested for each sample, and the average value was taken.

[0171] 4) Porosity

[0172] The porosity of the self-supporting electrode membrane was determined using a gravimetric method. First, the thickness of the electrode membrane was measured with a micrometer (accuracy ±1 μm), and the average value of five measurement points was recorded. Then, the mass of a membrane with a known area was weighed, and the apparent density was calculated. The theoretical density of the self-supporting electrode membrane was calculated according to the mixing rules based on the mass fraction of each component and its true density. Porosity P (%) = (1 - apparent density / theoretical density) × 100%. The membrane thickness (μm) and areal density (mg / cm³) were recorded simultaneously. 2 ).

[0173] 5) Average fiber length and fiber density

[0174] Representative areas were selected from the cross-section and surface of the self-supporting electrode film samples, and the fiber network formed by the second binder was observed using scanning electron microscopy (SEM). At least five fields of view were selected for each sample, and image analysis was performed on the resolvable continuous fibers to calculate the average fiber length. Simultaneously, the number of resolvable fibers per unit area was counted based on the field of view area, and the fiber density (fibers / μm²) was calculated. 2 For samples that cannot form a continuous self-supporting electrode film, are severely cracked and powdery, or cannot obtain a representative SEM field of view, the average fiber length and fiber density are denoted as " / ".

[0175] 6) Initial discharge specific capacity and initial coulombic efficiency

[0176] Using the aforementioned 3-5Ah LFP / graphite laminated pouch cells, tests were conducted at 25°C with the first 0.1C / 0.1C cycle after activation. The cells were charged at a constant current of 0.1C to the upper limit voltage of 3.65V, then charged at a constant voltage to a current of 0.05C, and finally discharged at a constant current of 0.1C to the lower limit voltage of 2.5V. The charge and discharge capacities of this cycle were recorded. The initial discharge specific capacity (mAh / g) was calculated as: discharge capacity / mass of positive electrode active material. The initial coulombic efficiency CE (%) was calculated as: (discharge capacity / charge capacity) × 100%.

[0177] 7) Ratio Capacity Retention Rate

[0178] At 25℃, the electrode was charged to 3.65V at a constant current and constant voltage of 0.1C (with a constant voltage cutoff current of 0.05C), and then discharged to 2.5V at constant currents of 0.1C and 0.5C, respectively. The average value was taken after three cycles at each rate. Using the 0.1C discharge capacity as a baseline, the 0.5C discharge capacity retention rate (%) was calculated as (C_0.5C / C_0.1C) × 100%. Rate capacity retention rate is an indicator for evaluating the polarization and ion transport capability of a thick-film electrode at higher current densities.

[0179] 8) Cyclic capacity retention rate at 25℃

[0180] Long-cycle stability testing was conducted at 25°C. The circuit was charged at a constant current of 0.5C to the upper limit voltage of 3.65V, then charged at a constant voltage to a current of 0.05C, and finally discharged at a constant current of 0.5C to the lower limit voltage of 2.5V. This process was repeated 500 times. The discharge capacity of the 500th cycle was recorded, and the capacity retention rate (%) was calculated as (500th cycle discharge capacity / 1st cycle discharge capacity) × 100%.

[0181] 9) High-temperature cycling capacity retention rate at 45℃

[0182] At 45℃, a high-temperature cycling test was conducted using the same charge and discharge regime as at 25℃. The cycle was repeated 200 times. The discharge capacity of the 200th cycle was recorded, and the capacity retention rate (%) was calculated as (200th discharge capacity / 1st discharge capacity) × 100%.

[0183] 10) DC internal resistance (DCIR) at 25℃

[0184] At 25°C, adjust the battery to 50% SOC and let it stand for 1 hour to allow the voltage to stabilize. Apply a 0.5C discharge pulse for 10 seconds, and record the open-circuit voltage V0 before the pulse starts and the terminal voltage V1 at the end of the pulse. Calculate the DC internal resistance (milliohms) according to DCIR=(V0-V1) / I, where I is the discharge pulse current (A). DCIR comprehensively reflects the electronic conductivity, ion transport, and interfacial contact quality of the electrodes.

[0185] The test results are shown in Tables 1 and 2.

[0186] Table 1

[0187]

[0188] Table 2

[0189]

[0190] As can be seen from the above, Examples 1 to 19, under the dual binder, two-step mixing, and roll-pressing composite conditions of this application, exhibited good overall film integrity, tensile strength, peel strength, rate performance, and cycle capacity retention. Examples 17 to 19 further optimized the fiber network by modifying the second binder, adding it in stages, or mixing it in stages during the second mixing process, resulting in improved performance compared to Example 1. In the comparative examples, the absence of the first binder, the absence of the second binder, one-time high-shear mixing, or exceeding the controlled mixing energy easily led to film failure, cracking, powder shedding, reduced peel strength, and decreased electrochemical performance. The above results indicate that this application, through the synergistic control of buffering with the first binder, moderate fiberization of the second binder, and guided by the conductive agent, can effectively suppress the over-fiberization problem of the dry-process lithium iron phosphate cathode, and improve the structural stability and battery performance of the dry-process self-supporting film.

[0191] As can be seen from the above description, the embodiments of the present invention achieve the following technical effects:

[0192] Based on an in-depth mechanistic analysis of the "over-fiberization" problem in lithium iron phosphate materials, this application proposes a technical solution that contrasts with existing high-nickel ternary dry electrode technologies. The core design of high-nickel ternary dry electrode technology lies in "how to promote sufficient fiberization," while the core design of this application lies in "how to control the degree of fiberization and prevent excessive fiberization." This shift in approach stems from a systematic study of the triple coupling effect of hardness, particle size, and surface properties in lithium iron phosphate materials. Therefore, this application constructs a ternary synergistic system guided by a second binder (fiberized binder) fiberized skeleton, a first binder (non-fiberized binder) filling buffer, and a conductive agent. The second binder undertakes the function of constructing the fiberized network skeleton, providing self-support for the electrode film. The first binder is dispersed in the interparticle gaps of the carbon-coated lithium iron phosphate material, acting as a "shear buffer layer" between hard particles and PTFE fibers during the mixing stage, thereby reducing local stress concentration. During the rolling stage, the first binder softens and forms a continuous point-bonded network, providing supplementary bonding force and repair capabilities for fiber breakage points. While constructing a conductive network, the conductive agent guides the displacement direction of lithium iron phosphate particles during the mixing process, promoting controlled directional fiber growth rather than random breakage. Specifically, in step S1, the raw materials of carbon-coated lithium iron phosphate material, the first binder, and the conductive agent are first mixed, and the equivalent shear rate of the first mixture is controlled within the aforementioned range. This allows these components to be premixed under low shear conditions, achieving uniform dispersion of each component. In particular, it ensures that the first binder is uniformly dispersed in the gaps between LFP particles, forming a preliminary "buffer layer" distribution. At the same time, the conductive agent particles or fibers must be uniformly wrapped on the surface of the LFP, and the fiberization of the second binder must be avoided at this stage. In step S2, the second binder is added separately to the first mixture, and fiberization mixing is performed using a relatively high shear force. The fiberization mixing energy required for the LFP system needs to be lower than that for the high-nickel ternary system because the hard LFP particles themselves provide efficient shear transfer for the fiberization process. Controlling the specific mixing energy of the second mixture within the aforementioned range can prevent the second binder from becoming over-fiberized. A self-supporting electrode film is formed by roll forming, thereby better combining with the current collector to form a positive electrode sheet. Therefore, the preparation method of this application can simultaneously optimize tensile strength, self-support and electrochemical activity under the harsh conditions of high hardness and small particle size of LFP materials, thereby improving the electrochemical performance of battery cells and better meeting the application scenarios of large-capacity and long-cycle-life batteries.

[0193] The above are merely embodiments of the present invention and are 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 battery cell, comprising preparing a positive electrode sheet, and sequentially stacking and assembling the positive electrode sheet, a separator, and a negative electrode sheet to obtain the battery cell, characterized in that, The method for preparing the positive electrode includes: Step S1: The raw materials, including carbon-coated lithium iron phosphate material, a first binder, and a conductive agent, are first mixed to obtain a first mixture; the equivalent shear rate of the first mixture is <10s. -1 ; Step S2: The raw materials including the first mixture and the second binder are fiberized by a second mixing process to obtain a second mixture; the equivalent shear rate of the second mixture is greater than the equivalent shear rate of the first mixture, and the specific mixing energy of the second mixture is 0.01~0.5kWh / kg. Step S3: Roll the second mixture into a film to obtain a self-supporting electrode film; Step S4: Combine the self-supporting electrode film and the current collector to obtain the positive electrode sheet; The second adhesive includes polytetrafluoroethylene; the first adhesive is polyvinylidene fluoride and / or polyvinylidene fluoride-hexafluoropropylene copolymer.

2. The method for preparing a single battery cell according to claim 1, characterized in that, In step S1, the temperature of the first mixing is 25~50℃, the mixing time is 5~30min, and the equivalent shear rate of the first mixing is 1~9s. -1 .

3. The method for preparing a single battery cell according to claim 1, characterized in that, In step S1, the mass ratio of the carbon-coated lithium iron phosphate material, the first binder, and the conductive agent is 86~98:0.5~6:1~8; And / or, the D50 particle size of the primary particles of the carbon-coated lithium iron phosphate material is 0.5~5μm, and the BET specific surface area of ​​the carbon-coated lithium iron phosphate material is 8~25m². 2 The tap density of the carbon-coated lithium iron phosphate material is 0.8~1.5 g / cm³. 3 The carbon coating layer on the surface of the carbon-coated lithium iron phosphate material has a mass content of 0.5~3wt%.

4. The method for preparing a single battery cell according to claim 1, characterized in that, In step S1, the D50 particle size of the first adhesive is 1~50μm, and the weight-average molecular weight of the first adhesive is 200000~800000g / mol. And / or, the conductive agent is selected from any one or more of carbon black, carbon nanotubes, carbon fibers, and graphene; the aspect ratio of the carbon nanotubes is ≥50.

5. The method for preparing a battery cell according to any one of claims 1 to 4, characterized in that, In step S2, the equivalent shear rate of the second mixture is 10~200s. -1 The temperature of the second mixing is 25~90℃, the specific mixing energy of the second mixing is 0.02~0.2kWh / kg, and the mixing time is 5~60min.

6. The method for preparing a battery cell according to any one of claims 1 to 4, characterized in that, In step S2, the mass ratio of the second adhesive to the first adhesive is (1:4) to (4:1). And / or, the D50 particle size of the second adhesive is 100~500μm, and the density of the second adhesive is 2.0~2.3g / cm³. 3 .

7. The method for preparing a battery cell according to any one of claims 1 to 4, characterized in that, In step S2, the second binder is a modified second binder with inorganic nanoparticles loaded on its surface. The inorganic nanoparticles are silicon dioxide and / or aluminum oxide, and the loading amount of the inorganic nanoparticles is 0.05~0.5wt%.

8. The method for preparing a battery cell according to any one of claims 1 to 4, characterized in that, In step S2, the second binder in the second mixture is added in stages and multiple times, with each addition amounting to 20-40% of the total mass of the second binder, and the interval between two adjacent additions being 2-10 minutes. And / or, in step S2, the second mixing process includes a third mixing and a fourth mixing performed sequentially; the equivalent shear rate of the third mixing is 10~80s. -1 The temperature of the third mixing is 25~70℃, the specific mixing energy of the third mixing is 0.01~0.1kWh / kg, and the time of the third mixing accounts for 20~50% of the total time of the second mixing; The equivalent shear rate of the fourth mixture is 10-120 s higher than that of the third mixture. -1 The temperature of the fourth mixture is 5-30°C higher than that of the third mixture, and the specific mixing energy of the fourth mixture is 0.01-0.15 kWh / kg higher than that of the third mixture.

9. The method for preparing a battery cell according to any one of claims 1 to 4, characterized in that, The temperature of the roll forming process is 25~120℃, the roll forming line pressure is 5~50MPa, and the number of roll forming passes is 1~5. And / or, the composite method is hot pressing, the hot pressing temperature is 100~200℃, the hot pressing pressure is 1~20MPa, and the hot pressing time is 1~30s; and / or, the current collector is aluminum foil and / or carbon-coated aluminum foil.

10. A battery cell, comprising a positive electrode, a separator, and a negative electrode, wherein the positive electrode comprises a current collector and a self-supporting electrode film, characterized in that, The battery cell is prepared by the preparation method according to any one of claims 1 to 9.

11. The battery cell according to claim 10, characterized in that, The self-supporting electrode film comprises carbon-coated lithium iron phosphate material, a second binder, a first binder, and a conductive agent; The second adhesive accounts for 0.5% to 5% of the total mass of the self-supporting electrode film. The mass of the first adhesive accounts for 0.5-6% of the total mass of the self-supporting electrode film; The mass of the conductive agent accounts for 1 to 8% of the total mass of the self-supporting electrode film.

12. The battery cell according to claim 10, characterized in that, The second binder forms a three-dimensional continuous fiber network structure in the self-supporting electrode film, wherein the average length of the fibers in the three-dimensional continuous fiber network is 10~200μm, and the fiber density of the three-dimensional continuous fiber network is 1~20 fibers / μm. 2 ; And / or, the thickness of the current collector is 10~20μm; and / or, the thickness of the self-supporting electrode film is 100~500μm, the tensile strength of the self-supporting electrode film is 3~30MPa, the porosity of the self-supporting electrode film is 20~50%, and the areal density of the self-supporting electrode film is 15~50mg / cm³. 2 .

13. A battery device, characterized in that, The battery device includes the battery cell according to any one of claims 10 to 12, and the battery device includes any one or more of the following: battery module, battery pack, and energy storage battery.

14. An electrical appliance, characterized in that, The electrical device includes the battery device of claim 13, the battery device being used to provide electrical energy.

15. An energy storage device, characterized in that, The energy storage device includes the battery device of claim 13, the battery device being used to store electrical energy.

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