High-stability lithium iron phosphate positive electrode slurry based on synergistic enhancement of carbon nanotubes and hectorite and preparation method of high-stability lithium iron phosphate positive electrode slurry

Abstract

The invention relates to high-stability lithium iron phosphate positive electrode slurry based on synergistic enhancement of carbon nanotubes and hectorite and a preparation method of the high-stability lithium iron phosphate positive electrode slurry, and belongs to the technical field of lithium iron phosphate battery positive electrode slurry. The positive electrode slurry comprises lithium iron phosphate, binder slurry and a conductive compounding agent, the conductive compounding agent is a mixture of a carbon nano tube, conductive carbon black and hectorite, and the mass ratio of the carbon nano tube to the conductive carbon black to the hectorite is as follows: the carbon nano tube to the conductive carbon black to the hectorite is (2.0-X): X: (0.3-0.5); and X is equal to 0.8 to 1.0. Through multi-dimensional collaborative innovation, the technical bottlenecks of insufficient conductivity, poor stability, complex additive, high settlement risk and the like in the traditional LFP positive electrode slurry are effectively overcome, the electrochemical performance and the energy density of the battery are improved, meanwhile, the process simplicity and economy are considered, and the preparation method has wide application prospects and industrialization value.

Description

Technical Field

[0001] This invention relates to a positive electrode slurry for lithium-ion batteries, and more particularly to a highly stable lithium iron phosphate positive electrode slurry based on the synergistic reinforcement of carbon nanotubes and lithium saponite, and its preparation method. It belongs to the technical field of lithium iron phosphate battery positive electrode slurry. Background Technology

[0002] Lithium-ion batteries, as core energy storage devices in the current new energy field, are widely used in electric vehicles, energy storage power stations, power tools, and portable electronic devices. The cathode material, as the main source of lithium ions and electrons in the battery, directly affects the battery's energy density, rate performance, cycle life, and safety. Among many cathode materials, lithium iron phosphate (LiFePO4, LFP) has become an important choice in the power and energy storage battery fields due to its excellent thermal stability, high safety, long cycle life, and relatively low cost. However, LFP materials have low intrinsic electronic conductivity (approximately 10⁻⁶). -9 The slow lithium-ion diffusion rate (S / cm) severely limits its performance under high-rate charge-discharge conditions. Therefore, to improve its electrochemical performance, it is usually necessary to add conductive agents to the positive electrode slurry to construct an efficient electron transport network. At the same time, a matching binder and solvent system should be selected to ensure that the active material, conductive agent, etc. are uniformly dispersed and well combined in the slurry and remain stable during the electrode preparation process.

[0003] Based on the different types of binders and solvent properties, the mainstream lithium iron phosphate cathode slurry systems in the industry can be mainly divided into the following categories.

[0004] I. Conventional oil-based system: LFP - conductive agent - PVDF - NMP

[0005] This is currently the most mature and widely used cathode slurry system in the industry, especially suitable for LFP and ternary oxide cathode materials. Polyvinylidene fluoride (PVDF) is a high-performance fluoropolymer binder with excellent chemical and thermal stability, good adhesion and film-forming properties, and can form a stable conductive and structural support network in the electrode. N-methylpyrrolidone (NMP) is a polar aprotic solvent with excellent solubility for PVDF, forming a uniform and stable gel, which is beneficial for the dispersion of active materials and conductive agents. The NMP solvent system has mature technology and stable coating performance, making it the preferred solvent for industrial production. The advantages of this system are mature technology, strong equipment versatility, suitability for large-scale production; good electrode uniformity after coating and drying, high bonding strength, and good flexibility; and good compatibility with LFP and various conductive agents and binders. The disadvantages are that NMP solvent has a certain degree of toxicity, posing potential risks to operator health and the environment, and has high usage and disposal costs. To ensure conductivity, a large amount of conductive agent (such as conductive carbon black SP and carbon nanotubes CNT) is usually added, which leads to a decrease in the proportion of active material and affects energy density. Traditional conductive agent systems are prone to agglomeration and sedimentation in the slurry, resulting in poor slurry stability and affecting coating and electrode quality. In this system, to improve the electronic conductivity of LFP, the industry usually uses a conductive agent system composed of SP and CNT, but the total amount of conductive agent used is relatively high, and it is difficult to ensure excellent conductivity while also taking into account slurry stability and process simplicity.

[0006] II. Aqueous System: LFP - Conductive Agent - Aqueous Binder - Deionized Water

[0007] This system has gained increasing attention in recent years due to environmental and cost considerations, but it still has certain limitations in LFP applications. The binder often uses a composite system of styrene-butadiene rubber (SBR) and carboxymethyl cellulose (CMC), where CMC provides thickening and suspension stabilization, and SBR provides bonding properties. The advantages of this system are that it does not use organic solvents, aligning with sustainable development trends; and it is relatively low-cost, suitable for cost-sensitive applications. The disadvantages are that the interfacial bonding between water-based binders and LFP is generally weaker than that of PVDF, resulting in poor interfacial stability under high temperature or pressure; water may cause some corrosion to the LFP surface, affecting the material structure and performance; the electrode flexibility and electrolyte resistance are relatively weak, limiting cycle life; and the conductive agent is difficult to disperse in water, easily settling, requiring higher slurry stability. Currently, water-based systems have not yet become mainstream in LFP, mainly used in scenarios with high cost and environmental requirements but relatively relaxed performance requirements.

[0008] III. Other Special Systems

[0009] Although some studies have attempted to use PC as a solvent in conjunction with certain special binders for positive electrode slurries, PC has almost no practical application value in LFP applications due to its poor wettability with LFP, poor compatibility with commonly used binders such as PVDF, and the potential for side reactions.

[0010] Non-polar / weakly polar solvent systems, such as alcohols and alkanes, have poor solubility in PVDF and weak dispersion ability for conductive agents, and are therefore rarely used in LFP cathode slurries.

[0011] The invention patent with authorization announcement number CN 112002950 B discloses a lithium-ion battery cathode slurry that uses a compound of polyacrylic acid and nano-lithium magnesium silicate as an additive, and claims to improve the stability and processing performance of the slurry by adjusting its rheological properties and interfacial interactions. However, this technical solution has the following obvious shortcomings: 1) System mismatch and disconnect from mainstream processes: The polyacrylic acid and nano-lithium magnesium silicate compound in this solution is a polar protic solvent system based on deionized water, which is incompatible with the PVDF / NMP system widely used in the current industry; 2) Conductivity problem not fundamentally solved: This solution does not propose an effective solution to the inherent problem of low electronic conductivity of LFP materials, and still relies on a high proportion of conductive agent to maintain the conductive network, failing to achieve excellent electronic transport performance while reducing the amount of conductive agent used. Summary of the Invention

[0012] To solve the above problems, the present invention provides, in a first aspect, a highly stable lithium iron phosphate cathode slurry based on the synergistic enhancement of carbon nanotubes and lithium saponite.

[0013] The technical solution of the present invention to solve the above problems is as follows:

[0014] A highly stable lithium iron phosphate cathode slurry based on the synergistic enhancement of carbon nanotubes and lithium saponite includes lithium iron phosphate, binder slurry, and conductive compounding agent; the conductive compounding agent is a mixture of carbon nanotubes, conductive carbon black, and lithium saponite, and the mass ratio of carbon nanotubes, conductive carbon black, and lithium saponite is: carbon nanotubes: conductive carbon black: lithium saponite = (2.0-X):X:(0.3~0.5); X = 0.8~1.0.

[0015] To address key issues in existing technologies, such as insufficient compatibility between slurry components and the matrix interface, low efficiency in constructing electronic conductive networks, and poor synergy of functional additives, this invention proposes a technical solution based on the research approach of "interface regulation - multi-scale conductive network construction - synergistic optimization of rheological behavior." Starting from the component matching design of the slurry system and the synergistic enhancement mechanism of functional materials, this invention innovatively introduces the synergistic enhancement of lithium saponite and carbon nanotubes based on the classic "LFP-SP-CNT-PVDF-NMP" system.

[0016] This scheme, while strictly adhering to the mainstream industrial process (i.e., using PVDF as a polyvinylidene fluoride binder and NMP as a highly polar aprotic solvent), achieves a triple synergistic optimization of improved electronic conductivity, enhanced slurry rheological stability, and simplified preparation process through precise control of the conductive agent system (reducing the total amount added) and the targeted introduction of functional thixotropic additives (lithium saponite). Its core mechanism includes the following three synergistic effects:

[0017] (1) Precise construction of multi-scale conductive networks

[0018] Carbon nanotubes: With their linear nanostructure, they form continuous electron transport channels that span three-dimensional space in the slurry system. By reducing the Schottky barrier and contact resistance between active material (LFP) particles, they significantly improve the interfacial electron conduction efficiency, thereby improving rate performance (capacity retention at high current density).

[0019] Conductive carbon black (SP): As zero-dimensional nanoparticles (particle size of about 20~50 nm), it forms dense short-range conductive nodes in the gaps of CNTs through its high surface energy properties, fills the structural gaps of the CNT network, enhances the electron hopping conduction path in local areas, and together with CNTs, constructs a hierarchical conductive network of "long-range continuous + short-range dense", improving the uniformity and reliability of overall electron transport.

[0020] Synergistic effect: CNTs and SPs form complementary conductive paths through spatial complementary distribution (interweaving of one-dimensional and zero-dimensional structures), avoiding local insulating regions caused by the aggregation or uneven distribution of a single conductive agent. Thus, the electronic conductivity of the system is maintained or even enhanced while reducing the total amount of conductive agent used.

[0021] (2) Rheology-structure synergistic regulation of functional thixotropic additives (lithium saponite)

[0022] Layered structure and spatial barrier effect: Lithium saponite, as a natural two-dimensional layered silicate mineral (interlayer spacing of about 1.0~1.5 nm, specific surface area >300 m² / g), has a lamellar structure that can be adsorbed onto the surface of CNT / SP particles through van der Waals forces and electrostatic interactions, forming a physical barrier layer. This effectively inhibits the agglomeration and gravitational sedimentation of conductive agent particles, and maintains the uniformity of dispersion of active substances and conductive agents in the slurry.

[0023] Thixotropic thickening and rheological stability: Lithium saponite forms a reversible gel network in NMP solvent through interlayer intercalation, giving the slurry high viscoelasticity. Under static conditions, the tight packing of the interlayer structure inhibits particle sedimentation. Under shear forces (such as during coating), the apparent viscosity is reduced through interlayer slip, ensuring that the slurry has both good storage stability (no stratification / hard precipitation) and processing fluidity (low viscosity coating adaptability).

[0024] Enhanced uniformity of conductive network distribution: The interlayer nanoscale size of lithium saponite is highly compatible with the diameter of single-walled / few-walled CNTs. Under mechanical stirring (e.g., high-speed dispersion at 2000~2500 rpm) during slurry preparation, single-walled / few-walled CNTs can be embedded into the interlayer structure of lithium saponite, forming a "CNT@lithium saponite" intercalation composite. This composite, on the one hand, restricts the random aggregation of CNTs through the layered framework of lithium saponite, and on the other hand, after slurry solidification, guides the distribution of CNTs along a specific direction through the directional arrangement of the interlayer structure, thereby improving the spatial uniformity and structural stability of the conductive network and further enhancing the isotropic nature of electron transport.

[0025] (3) Synergistic balance between process and performance

[0026] This solution maintains the industrial applicability of the PVDF / NMP system (without requiring replacement of binders or solvents), while maximizing the proportion of active material (LFP) (≥95 wt%) by reducing the total amount of conductive agent (approximately 20-30% less than traditional solutions) and integrating the functions of lithium saponite (replacing some rheology modifiers), thereby improving the energy density of the battery. At the same time, through high-speed dispersion and vacuum degassing processes, the complex process of adding multiple additives in steps in traditional solutions is simplified, significantly reducing the difficulty of process control and production costs.

[0027] As a preferred embodiment of the above technical solution, the binder slurry is composed of PVDF and NMP, and the mass ratio of PVDF to lithium saponite is lithium saponite:PVDF=(0.3~0.5):(1.8~2.2); the amount of NMP is such that the solid content of the positive electrode slurry reaches 65~75%.

[0028] As a preferred embodiment of the above technical solution, the mass ratio of lithium iron phosphate to conductive compound is lithium iron phosphate : conductive compound = (95~97) : (2.3~2.5).

[0029] In the above-mentioned preferred technical solution, the conductive compounding agent accounts for less than 2.5 wt% of the total mass of the slurry, which is significantly lower than that of the traditional solution (the conductive agent usually accounts for 3~5 wt%), thereby increasing the effective loading of LFP per unit mass of slurry and improving the energy density and specific capacity of the battery.

[0030] Secondly, a method for preparing the aforementioned positive electrode slurry is provided.

[0031] A method for preparing a highly stable lithium iron phosphate cathode slurry based on the synergistic reinforcement of carbon nanotubes and lithium saponite includes the following steps:

[0032] S1. The raw materials are prepared in the following mass ratio: lithium iron phosphate: carbon nanotubes: conductive carbon black: lithium saponite: PVDF = (95~97):(2.0-X):X:(0.3~0.5):(1.8~2.2).

[0033] S2. Slowly add PVDF powder to NMP solvent and stir until completely dissolved to form a transparent adhesive solution;

[0034] S3. The carbon nanotubes, conductive carbon black and lithium saponite are premixed and then added to the transparent adhesive solution. The mixture is dispersed at a first speed of not less than 1500 rpm to obtain a dispersion slurry.

[0035] S4. Under stirring at the second rotation speed, the lithium iron phosphate is added to the dispersion slurry until the slurry is uniform; thus, a positive electrode preparation slurry is obtained.

[0036] S5. The positive electrode preparation slurry is subjected to a vacuum environment and stirred at the third rotation speed to remove air bubbles, and then allowed to stand and age for more than 8 hours to obtain the high-stability lithium iron phosphate positive electrode slurry.

[0037] In the above-mentioned technical solution of the present invention, the four-step process of "premixing-high-speed dispersion-main mixing-defoaming and aging" ensures that CNT / SP / lithium saponite is uniformly dispersed in PVDF adhesive, and then fully mixed with LFP to finally form a stable slurry with no agglomeration, no sedimentation and few bubble defects, laying the foundation for uniform coating of electrode sheets.

[0038] As a preferred embodiment of the above technical solution, in step S2, PVDF powder is slowly added to a portion of the NMP solvent, and in any step S3 to S5, the remaining NMP solvent is added to the system until the solid content of the positive electrode slurry reaches 65 to 75%.

[0039] As a preferred embodiment of the above technical solution, in step S5, the mixture is stirred at a third rotation speed not exceeding 1000 rpm under a relative vacuum degree at least 0.090 MPa lower than the standard atmospheric pressure.

[0040] In the above-mentioned technical solution of the present invention, degassing is performed by low-speed stirring (≤1000 rpm) in a vacuum environment (≤-0.090 MPa). This avoids the introduction of new air bubbles by high-speed shearing while effectively removing trapped air from the slurry, preventing defects such as pinholes and delamination during electrode preparation, and improving electrode consistency. At the same time, low-speed stirring promotes further compact packing of slurry components, enhancing the overall stability of the slurry.

[0041] As a preferred embodiment of the above technical solution, the ambient temperature in steps S2 to S5 is 20 to 30°C.

[0042] Excessive temperature may cause NMP to evaporate too quickly (affecting solids content and viscosity) and PVDF to dissolve abnormally; excessively low temperature may reduce dispersion efficiency and the thixotropic activity of lithium saponite. Maintaining a constant temperature of 20-30℃ ensures consistency in the dissolution, dispersion, and reaction behavior of each component, guaranteeing batch-to-batch repeatability of the slurry performance.

[0043] As a preferred embodiment of the above technical solution, in step S3, a dry powder mixer is used for premixing.

[0044] As a preferred embodiment of the above technical solution, in step S3, the first rotational speed is 2000~2500 rpm.

[0045] Under high-intensity shear force (2000~2500 rpm), the nanoscale aggregates of CNT / SP are effectively dissociated; at the same time, the lamellar structure of lithium saponite is fully unfolded, forming a close physical interaction with CNT, achieving high dispersion and uniform distribution of the conductive agent, and constructing a stable conductive network.

[0046] As a preferred embodiment of the above technical solution, in step S3, the second rotational speed is 1000~1500 rpm.

[0047] LFP is added under medium-low speed shearing (1000~1500 rpm) to avoid high-speed stirring from destroying the existing conductive network structure. At the same time, it ensures that the LFP particles and the conductive compound are in full contact to form a uniform composite system of "active material-conductive agent-binder", and finally obtains a uniformly dispersed positive electrode preparative slurry.

[0048] In summary, the present invention has the following beneficial effects:

[0049] 1. This invention addresses key issues in existing lithium iron phosphate (LFP) cathode slurries, such as insufficient compatibility between slurry components and the matrix interface, low efficiency in building electronic conductive networks, and poor synergy of functional additives. Through a scientific design of "conductive agent hierarchical network construction - multi-effect regulation of functional thixotropic additives - precise matching of process parameters," it achieves a triple synergistic optimization of improved electronic conductivity, enhanced slurry rheological stability, and simplified preparation process. This breakthrough overcomes the technical bottlenecks of traditional LFP cathode slurries and provides key technical support for the large-scale preparation of high-energy-density, high-consistency lithium-ion batteries.

[0050] 2. This invention effectively solves the problems of uneven distribution and high interfacial resistance in traditional single conductive agents by constructing a "multi-scale hierarchical conductive network." Carbon nanotubes (CNTs), with their one-dimensional linear nanostructure, form continuous electron transport channels spanning three dimensions in the slurry system, significantly reducing the Schottky barrier and contact resistance between LFP particles and improving electron conduction efficiency under high current density. Conductive carbon black (SP), as zero-dimensional nanoparticles (approximately 20-50 nm in diameter), fills the gaps between CNTs with its high surface energy, forming dense short-range conductive nodes and enhancing electron hopping conduction paths in local areas. The two interact spatially to form a complementary conductive network, avoiding localized insulating regions caused by the agglomeration or uneven distribution of a single conductive agent.

[0051] 3. This invention achieves multi-effect regulation of slurry rheological behavior and structural stability through the directional introduction of functional thixotropic additives. Lithium saponite, a natural two-dimensional layered silicate mineral (interlayer spacing of approximately 1.0~1.5 nm, specific surface area >300 m² / g), has a lamellar structure that adsorbs onto the surface of CNT / SP particles through van der Waals forces and electrostatic interactions, forming a physical barrier layer that effectively inhibits the gravitational aggregation and sedimentation of conductive agent particles, maintaining the uniform dispersion of active substances and conductive agents in the slurry. Simultaneously, lithium saponite forms a reversible gel network in NMP solvent, endowing the slurry with high viscoelasticity—in the static state, the tight packing of interlayer structures inhibits particle sedimentation, and under shear forces (such as during coating), interlayer slip reduces apparent viscosity, ensuring that the slurry possesses both good storage stability (no stratification / hard precipitation) and processing fluidity (low viscosity coating adaptability). Furthermore, the interlayer nanoscale of lithium saponite is highly matched with the diameter of single-walled / few-walled CNTs. Under mechanical stirring, CNTs can be embedded in the interlayer of lithium saponite to form a "CNT@lithium saponite" intercalation complex, which further restricts CNT aggregation and guides its directional distribution, improves the spatial uniformity and structural stability of the conductive network, and thus enhances the mechanical strength and long-term cycle reliability of the electrode.

[0052] 4. This invention, while maintaining the classic oily system of PVDF / NMP (the mainstream industrial process), maximizes the proportion of active material (LFP) (≥95 wt%) through precise control of the conductive agent system (total dosage reduced to 2.3~2.5 wt%) and functional integration of lithium saponite (replacing some rheology modifiers). Compared with traditional solutions, this solution significantly reduces the proportion of inactive components, directly increasing the effective loading of LFP per unit mass of slurry, thereby improving the energy density and specific capacity of the battery.

[0053] 5. This invention is based on a mature PVDF / NMP system (requiring no change in binder or solvent). Through a standardized four-step process of "premixing - high-speed dispersion - main mixing - degassing and aging," combined with precise process parameter control (e.g., high-speed dispersion speed 2000~2500 rpm, vacuum degassing pressure ≤ -0.090 MPa, ambient temperature 20~30℃), it achieves the preparation of stable slurry with no agglomeration, no sedimentation, and few bubble defects. Specifically, the vacuum degassing process effectively removes entrained air under low-speed stirring (≤1000 rpm), preventing electrode pinholes and delamination defects. Dry powder premixing and stepwise speed control (premixing 2000~2500 rpm, main mixing 1000~1500 rpm) ensure uniform dispersion of the conductive agent and gentle mixing of LFP, avoiding damage to the conductive network. This process is compatible with existing production equipment, requiring no additional complex devices, significantly reducing process control difficulty and manufacturing costs, while ensuring batch-to-batch repeatability of slurry performance, demonstrating significant industrial feasibility.

[0054] 6. In summary, this invention, through multi-dimensional collaborative innovation, effectively overcomes the technical bottlenecks of insufficient conductivity, poor stability, complex additives, and high sedimentation risk in traditional LFP cathode slurries. While improving the electrochemical performance and energy density of the battery, it also takes into account the simplicity and economy of the process, and has broad application prospects and industrialization value. Attached Figure Description

[0055] Figure 1 The graph shows the three-stage slurry recovery rate of Example 1.

[0056] Figure 2 The curve of the three-stage slurry recovery rate is shown in Comparative Example 3.

[0057] Figure 3 The graph shows the three-stage slurry recovery rate of Comparative Example 4. Detailed Implementation

[0058] The present invention will be further explained and described below with reference to the accompanying drawings.

[0059] This specific embodiment is merely an explanation of the present invention and is not intended to limit the invention. Any changes made by those skilled in the art after reading this specification, as long as they fall within the scope of the claims, will be protected by patent law.

[0060] Example 1

[0061] A highly stable lithium iron phosphate cathode slurry based on the synergistic enhancement of carbon nanotubes and lithium saponite comprises lithium iron phosphate, a binder slurry, and a conductive compounding agent. The conductive compounding agent is a mixture of carbon nanotubes, conductive carbon black, and lithium saponite. The mass ratio of each component is LFP∶SP∶CNT∶PVDF∶lithium saponite = 96.0∶1.2∶0.8∶2.0∶0.4. Solid content control: The amount of NMP used is adjusted to achieve a solid content of 70% in the cathode slurry.

[0062] Specifically, the preparation process of this positive electrode slurry includes the following steps:

[0063] S1. Prepare the raw materials as required;

[0064] S2, PVDF adhesive preparation:

[0065] Slowly add PVDF powder to a portion of NMP and stir at 25°C until completely dissolved to form a transparent liquid.

[0066] S3, Conductive compounding agent premix:

[0067] SP, CNT and lithium saponite were premixed using a dry powder mixer to obtain a premixed powder; the premixed powder was added to PVDF adhesive and dispersed at a high speed of 2000 rpm for 60 minutes.

[0068] S4, Mixing of active substances:

[0069] Add LFP powder in batches, and switch to 1000 rpm and stir for 120 minutes until homogeneous;

[0070] S5, Defoaming and Aging:

[0071] Under a vacuum of -0.095 MPa, stir at a low speed of 500 rpm for 30 minutes to remove bubbles, and then let stand and age for 12 hours.

[0072] Example 2

[0073] A highly stable lithium iron phosphate cathode slurry based on the synergistic enhancement of carbon nanotubes and lithium saponite comprises lithium iron phosphate, a binder slurry, and a conductive compounding agent. The conductive compounding agent is a mixture of carbon nanotubes, conductive carbon black, and lithium saponite. The mass ratio of each component is LFP∶SP∶CNT∶PVDF∶lithium saponite = 96.0∶1.0∶1.0∶2.0∶0.4. Solid content control: The amount of NMP used is sufficient to achieve a solid content of 70% in the cathode slurry.

[0074] The preparation process is the same as in Example 1.

[0075] Example 3

[0076] A highly stable lithium iron phosphate cathode slurry based on the synergistic enhancement of carbon nanotubes and lithium saponite comprises lithium iron phosphate, a binder slurry, and a conductive compounding agent. The conductive compounding agent is a mixture of carbon nanotubes, conductive carbon black, and lithium saponite. The mass ratio of each component is LFP∶SP∶CNT∶PVDF∶lithium saponite = 96.0∶1.2∶0.8∶2.0∶0.4. Solid content control: The amount of NMP used achieves a solid content of 65% in the cathode slurry.

[0077] The preparation process is the same as in Example 1.

[0078] Example 4

[0079] A highly stable lithium iron phosphate cathode slurry based on the synergistic enhancement of carbon nanotubes and lithium saponite comprises lithium iron phosphate, a binder slurry, and a conductive compounding agent. The conductive compounding agent is a mixture of carbon nanotubes, conductive carbon black, and lithium saponite. The mass ratio of each component is LFP∶SP∶CNT∶PVDF∶lithium saponite = 96.0∶1.2∶0.8∶2.0∶0.4. Solid content control: The amount of NMP used achieves a solid content of 75% in the cathode slurry.

[0080] The preparation process is the same as in Example 1.

[0081] Example 5

[0082] A highly stable lithium iron phosphate cathode slurry based on the synergistic enhancement of carbon nanotubes and lithium saponite comprises lithium iron phosphate, a binder slurry, and a conductive compounding agent. The conductive compounding agent is a mixture of carbon nanotubes, conductive carbon black, and lithium saponite. The mass ratio of each component is LFP∶SP∶CNT∶PVDF∶lithium saponite = 96.0∶1.0∶1.0∶1.8∶0.5. Solid content control: The amount of NMP used is sufficient to achieve a solid content of 70% in the cathode slurry.

[0083] The preparation process is the same as in Example 1.

[0084] Comparative Example 1 (Traditional Formula: CNT and Lithium Soapstone Free)

[0085] The formulation is LFP∶SP∶PVDF = 96.5∶2.0∶1.5 (containing only conductive carbon black, without carbon nanotubes and lithium saponite). Solid content control: NMP dosage ensures the positive electrode slurry has a solid content of 70%.

[0086] The preparation process is as follows:

[0087] S1. Prepare the raw materials as required;

[0088] S2, PVDF adhesive preparation:

[0089] Slowly add PVDF powder to a portion of NMP and stir at 25°C until completely dissolved to form a transparent liquid.

[0090] S3. Slurry preparation:

[0091] LFP and SP were premixed using a dry powder mixer to obtain a premixed powder; the premixed powder was added to PVDF adhesive and dispersed at 2000 rpm for 120 minutes until uniform.

[0092] S4. Defoaming and aging:

[0093] Under a vacuum of -0.095 MPa, stir at a low speed of 500 rpm for 30 minutes to remove bubbles, and then let stand and age for 12 hours.

[0094] Comparative Example 2 (Conductivity Enhancement Formula: Contains CNTs but does not contain lithium soapstone)

[0095] The formula is LFP∶SP∶CNT∶PVDF = 96.0∶1.2∶0.8∶2.0 (containing carbon nanotubes but without lithium saponite).

[0096] Solid content control: The amount of NMP used is such that the solid content of the cathode slurry is 70%.

[0097] The preparation process is as follows:

[0098] S1. Prepare the raw materials as required;

[0099] S2, PVDF adhesive preparation:

[0100] Slowly add PVDF powder to a portion of NMP and stir at 25°C until completely dissolved to form a transparent liquid.

[0101] S3. Slurry preparation:

[0102] LFP, SP and CNT were premixed using a dry powder mixer to obtain a premixed powder; the premixed powder was added to PVDF adhesive and dispersed at 2000 rpm for 120 minutes until uniform.

[0103] S4. Defoaming and aging:

[0104] Under a vacuum of -0.095 MPa, stir at a low speed of 500 rpm for 30 minutes to remove bubbles, and then let stand and age for 12 hours.

[0105] Comparative Example 3 (complete formulation, but different preparation process)

[0106] The formula is LFP∶SP∶CNT∶PVDF∶Lithium soapstone = 96.0∶1.2∶0.8∶2.0∶0.4.

[0107] Solid content control: The amount of NMP used is such that the solid content of the cathode slurry is 70%.

[0108] The preparation process is as follows:

[0109] S1. Prepare the raw materials as required;

[0110] S2, PVDF adhesive preparation:

[0111] Slowly add PVDF powder to a portion of NMP and stir at 25°C until completely dissolved to form a transparent liquid.

[0112] S3. Slurry preparation:

[0113] LFP, SP, CNT and lithium saponite were premixed using a dry powder mixer to obtain a premixed powder; the premixed powder was added to PVDF adhesive and dispersed at 2000 rpm for 120 minutes until uniform.

[0114] S4. Defoaming and aging:

[0115] Under a vacuum of -0.095 MPa, stir at a low speed of 500 rpm for 30 minutes to remove bubbles, and then let stand and age for 12 hours.

[0116] Comparative Example 4 (complete formulation, but different preparation process)

[0117] The formula is LFP∶SP∶CNT∶PVDF∶Lithium soapstone = 96.0∶1.2∶0.8∶2.0∶0.4.

[0118] Solid content control: The amount of NMP used is such that the solid content of the cathode slurry is 70%.

[0119] The preparation process is as follows:

[0120] S1. Prepare the raw materials as required;

[0121] S2, PVDF adhesive preparation:

[0122] PVDF powder was slowly added to a portion of NMP and stirred at 25°C until completely dissolved to form a PVDF solution. Lithium saponite was added to the PVDF solution and dispersed at 2000 rpm for 60 minutes until homogeneous to obtain a transparent solution.

[0123] S3. Slurry preparation:

[0124] LFP, SP and CNT were premixed using a dry powder mixer to obtain a premixed powder; the premixed powder was added to a transparent adhesive liquid and dispersed at a high speed of 2000 rpm for 120 minutes until uniform.

[0125] S4. Defoaming and aging:

[0126] Under a vacuum of -0.095 MPa, stir at a low speed of 500 rpm for 30 minutes to remove bubbles, and then let stand and age for 12 hours.

[0127] The relevant performance of Example 1 and Comparative Examples 1-4 were tested, and the results are shown in Tables 1-3. Figures 1-3 .

[0128] Table 1. Results of 12H suspension stability test of the cathode slurry in Examples 1 and Comparative Examples 3-4 Example 1 Comparative Example 3 Comparative Example 4 0h 11304 11087 11191 2 h 11619 11383 11493 4 h 11938 11716 11823 6 h 12205 12115 12225 8 h 12412 12583 12693 10 h 12859 13186 13292 12 h 12941 15185 14295 .

[0129] Table 2. Physical property determination of the positive electrode slurry in Examples 1 and 3-4 Test Project Example 1 Comparative Example 3 Comparative Example 4 Viscosity 11304 11087 11191 Solid content 70.5 69.7 70.8 Viscosity recovery percentage after 12 hours of standing (%) 14.5 36.9 27.7 Slurry settling after 12 hours Upper layer solids content 70.8%, lower layer solids content 71.1%, no settlement. Upper layer solids content 71.3%, lower layer solids content 71.4%, slight settlement. Upper layer solids content 72.1%, lower layer solids content 72.6%, slight settlement. Three-stage slurry recovery rate % 82.97 69.66 74.39 .

[0130] Table 3. Electrical performance test results of the positive electrode slurry in Examples 1 and 3-4. Test Project Example 1 Comparative Example 3 Comparative Example 4 Capacity retention % after 100 cycles at 45℃ 100.9 100.3 100.5 Capacity retention % after 300 cycles at 45℃ 99.1 98.0 98.7 Capacity retention % after 500 cycles at 45℃ 97.0 94.3 96.8 Capacity retention % after 800 cycles at 45℃ 94.7 90.0 92.3 Capacity retention % after 1000 cycles at 45℃ 91.9 85.8 87.8 .

[0131] To test the electrical performance of the positive electrode slurry, the positive electrode slurry must first be coated to form a positive electrode sheet, and then the positive electrode sheet is used to make a standard battery. The specific process for making the standard battery is as follows:

[0132] Raw material pretreatment → pulping → coating and drying → rolling and slitting → die cutting → winding / stacking → preheating → hot pressing → weighing → grouping → pre-welding → ultrasonic welding → laser welding (adapter sheet) → core assembly - wrapping with Mylar film → shelling → helium detection → baking → secondary helium detection → formation → capacity testing.

[0133] The specific details of each process are as follows:

[0134] Coating and drying

[0135] Coating: The positive electrode slurry is evenly coated onto the aluminum foil current collector using a coating machine. The coating thickness (e.g., 100μm), coating speed, and coating uniformity must be precisely controlled to ensure that there are no scratches, missed coatings, or uneven coating thickness on the electrode surface, so as to meet the battery capacity and rate performance requirements.

[0136] Drying: The coated electrode enters the drying oven, where the solvent is removed by gradient heating to obtain a dried positive electrode. The residual solvent content must be controlled during the drying process (generally <0.5%) to avoid electrode deformation or bubble defects caused by solvent evaporation in subsequent processes.

[0137] Rolling and slitting

[0138] Rolling: The dried positive electrode sheet is rolled by a double roller press. By adjusting the rolling pressure and rolling speed, the compaction density of the electrode material is increased, thereby enhancing the electrode conductivity and mechanical strength. It is necessary to ensure uniform compaction density and avoid local over-pressure (leading to cracks) or under-pressure (affecting energy density).

[0139] Slitting: Based on the battery design dimensions (such as cell width and length), a precision slitting machine is used to slit the rolled whole electrode sheet into electrode strips that meet the requirements. The slitting size accuracy must be controlled within ±0.3mm, and the edges must be smooth and burr-free to meet the assembly requirements of subsequent winding / stacking processes.

[0140] Die cutting

[0141] The slit electrode sheets are further processed into electrode units by a die-cutting machine. The die-cutting process must ensure that the cut is flat, free of powder or edge damage, to avoid the risk of internal short circuit in the battery due to burrs or debris.

[0142] Winding / Layering

[0143] The winding process is adopted: the die-cut positive electrode sheet, negative electrode sheet and separator are stacked in the order of "separator-negative electrode-separator-positive electrode" and then wound into a square battery core by a winding machine. The winding tension and alignment must be controlled (the deviation between the edge of the electrode sheet and the separator is <0.2mm) to ensure that the core structure is tight and free from looseness or misalignment.

[0144] preheating

[0145] The wound battery cell semi-finished product is placed in a preheating equipment and preheated at 60°C for 10 minutes to allow the electrode sheet and separator to initially adhere, reduce deformation during subsequent hot pressing, and improve interfacial bonding.

[0146] Hot pressing

[0147] After preheating, the battery cells are hot-pressed and shaped using a hot press (temperature 100℃, pressure 1.2MPa) to further compact the electrode sheets and separator, remove internal air, and reduce interface resistance. After hot pressing, the consistency of the battery cell thickness (deviation < ±0.1mm) needs to be checked to ensure smooth subsequent assembly into the casing.

[0148] Weighing

[0149] The cells after hot pressing are weighed using a high-precision electronic scale, and the weight of each cell is recorded (accurate to 0.01g). This weight serves as the basis for subsequent grouping (screening cells with matching capacity / internal resistance) to ensure consistency among cells in the same group.

[0150] Grouping

[0151] Based on the weighing results and preliminary electrical performance data (such as internal resistance and thickness), the cells are grouped and matched according to parameters such as capacity, internal resistance, and weight (e.g., deviation ≤3%) to ensure that the performance of cells within the same battery pack is consistent and to avoid the overall performance degradation or safety risks of the battery pack due to individual cell differences.

[0152] Pre-welding

[0153] After pairing, the battery cell tabs (positive / negative) are fixed to the adapter plate or current collector by spot welding or ultrasonic pre-welding, thus initially connecting the tabs to the external circuit. The pre-welding must ensure that the solder joints are firm, without any poor solder joints or bursts, to prevent the tabs from falling off in subsequent processes.

[0154] Ultrasonic welding

[0155] The pre-welded tabs and adapter pieces are further ultrasonically welded, using high-frequency vibration to bond metal molecules together and form a high-strength, low-resistance weld point. After welding, the tensile strength (≥5N) and resistance (≤5mΩ) of the weld point need to be tested to ensure conductivity reliability.

[0156] Laser welding (adapter plate)

[0157] The connection between the adapter plate and the battery casing (such as steel / aluminum casing) is made by laser welding, which uses a high-energy laser beam to melt the metal surface to form a sealed weld. The welding must ensure that the weld is continuous, free of pores or cracks, and avoid damaging the internal cells or separator.

[0158] Core-wrapped Mylar membrane

[0159] After welding, the battery cell is assembled with the casing (e.g., the core is placed into a square aluminum casing). Then, Mylar insulating film (high temperature resistant polyester film) is used to wrap the key parts of the battery cell (such as the base of the tabs and the edge of the casing) to prevent the battery cell from directly contacting the casing and causing a short circuit, while also providing buffering and insulation protection.

[0160] Shell

[0161] After the cells are combined and wrapped with Mylar film, they are inserted into the battery casing (such as aluminum-plastic film / steel casing / aluminum casing) by a robotic arm or manually. During the insertion process, it is necessary to ensure that the cells are centered and not tilted or squeezed to avoid deformation of the casing or damage to the internal electrodes.

[0162] helium detection

[0163] After the battery is installed, the casing is tested for sealing using a helium mass spectrometer. Helium is used as a tracer gas to detect any minor leaks in the casing. Cells that fail the helium test are discarded to ensure that the battery does not leak electrolyte or allow external moisture to enter during long-term storage.

[0164] bake

[0165] Batteries that pass the helium test are placed in a vacuum oven for baking. The purpose is to remove residual moisture inside the cell (target moisture content <50ppm) to prevent moisture from reacting with the electrolyte to generate HF that corrodes the electrodes or causes gas production.

[0166] Secondary helium detection

[0167] After baking, the battery undergoes another helium test to verify whether the sealing performance has changed due to high-temperature stress, ensuring the long-term reliability of the final battery.

[0168] form

[0169] The battery that has passed the second helium test is connected to the charging and discharging equipment for the first charge (formation process). Usually, a small current (such as 0.05C~0.1C) is used to activate the electrode material (such as Li⁺ embedded in graphite or positive electrode lattice) and form a stable solid electrolyte interphase (SEI) film. The formation process requires strict control of temperature (such as 25~45℃), voltage (such as 2.5V~4.2V) and current density to avoid SEI film instability leading to cycle life degradation.

[0170] Capacity

[0171] After formation, the battery is subjected to a full charge-discharge test with a high current (such as 0.5C~1C) to determine the initial actual capacity (accurate to ±1mAh).

[0172] Key quality control points:

[0173] The electrodes are dried, with moisture content ≤5 ppm, and monitored by a dew point meter;

[0174] Liquid injection and sealing, no leakage, uniform electrolyte distribution, helium detector, X-ray imaging;

[0175] Capacity screening, capacity deviation ≤3%, internal resistance qualified, charge and discharge test cabinet;

[0176] Safety testing includes needle penetration, compression, and overcharge tests, as well as environmental simulation test chambers.

[0177] The method for determining circulating capacity is as follows:

[0178] The charge-discharge cycle method involves repeatedly performing constant current charge-discharge cycles, recording the discharge capacity of each cycle, and calculating the capacity retention rate (capacity of the Nth cycle / initial capacity).

[0179] Standardized process:

[0180] Charging: Charge at a constant current of 0.5C until the cutoff voltage is reached, then switch to constant voltage until the current drops to 0.05C;

[0181] Discharge: Discharge at a constant current of 1C until termination.

[0182] The three-stage test method for slurry recovery rate is as follows:

[0183] Stage 1, shear rate 5~30 s -1 The duration is 60~120 s to simulate a static state and establish the initial structure;

[0184] Stage 2, shear rate 50~500 s -1 The duration is 5~30 seconds, simulating processing steps such as coating and pumping;

[0185] Stage 3, shear rate 5~30 s -1 The duration is 60~300 seconds, and the structural recovery ability is observed.

[0186] Note: The curves of shear stress (τ) and viscosity (η) over time were recorded using a rotational rheometer.

[0187] Figure 1 The graph shows the three-stage slurry recovery rate of Example 1.

[0188] Figure 2 The curve of the three-stage slurry recovery rate is shown in Comparative Example 3.

[0189] Figure 3 The graph shows the three-stage slurry recovery rate of Comparative Example 4.

[0190] Combine Tables 1 to 3. Figures 1-3 It can be seen that:

[0191] The suspension stability of the slurry was significantly improved: After 12 hours of standing, the viscosity of the slurry in Example 1 increased by only 14.5%, far lower than the 36.9% in Comparative Example 3 and 27.7% in Comparative Example 4. This indicates that the present invention effectively inhibits the agglomeration and sedimentation of the conductive agent through the synergistic effect of carbon nanotubes and lithium saponite, significantly improving the long-term storage stability of the slurry. Comparative Example 3 (containing CNTs but without lithium saponite) and Comparative Example 4 (improper process sequence) both showed relatively obvious sedimentation phenomena. In particular, the difference in solid content between the upper and lower layers of Comparative Example 4 increased further after 12 hours, indicating that the process sequence has a significant impact on the uniformity of the slurry.

[0192] Superior slurry rheological behavior and strong structural recovery ability: results from three-stage slurry recovery rate test ( Figures 1-3As can be seen, after undergoing the high-shear (simulated coating process) and low-shear recovery stages, the slurry structure recovery rate of Example 1 reached 82.97%, significantly higher than the 69.66% of Comparative Example 3 and 74.39% of Comparative Example 4. This indicates that the present invention, through the "premix-high-speed dispersion-low-speed degassing" process and the thixotropic regulation effect of lithium saponite, enables the slurry to quickly reconstruct a stable network structure during processing, ensuring coating uniformity and electrode consistency.

[0193] Excellent electrochemical performance and long cycle life: In high-temperature cycling tests at 45℃, Example 1 exhibited capacity retention rates of 100.9%, 99.1%, 97.0%, 94.7%, and 91.9% after 100, 300, 500, 800, and 1000 cycles, respectively, all significantly better than Comparative Examples 1-4. Particularly after 800 and 1000 cycles, Example 1 achieved capacity retention rates of 94.7% and 91.9%, respectively, while Comparative Example 1 (without CNTs and lithium saponite) only achieved 90.0% and 85.8%, and Comparative Examples 3 and 4 maintained only around 92.3% and 87.8%. This demonstrates that by constructing an efficient multi-scale conductive network and a stable slurry structure, this invention not only improves the initial conductivity of the battery but also significantly enhances its structural stability and capacity retention during long-term cycling.

[0194] With strong process adaptability and industrialization advantages, this invention achieves a lithium petroleum product (LFP) content of ≥95 wt% by precisely controlling the conductive agent ratio (total addition ≤2.5 wt%) and introducing multifunctional lithium saponite additives. This significantly simplifies the slurry preparation process (e.g., reducing the step-by-step addition of multiple additives) while increasing energy density, thus lowering process control difficulty and manufacturing costs. Furthermore, vacuum degassing and low-speed stirring effectively avoid bubble and sedimentation defects, ensuring high consistency and yield in electrode preparation.

[0195] The high-stability lithium iron phosphate cathode slurry and its preparation method based on the synergistic enhancement of carbon nanotubes and lithium saponite proposed in this invention successfully solves the technical problems of insufficient conductivity, poor stability, complex additives and high sedimentation risk in traditional LFP cathode slurries. It shows outstanding performance in improving battery energy density, cycle life and processing consistency, and has significant prospects for industrial application and promotion value.

Claims

1. A highly stable lithium iron phosphate cathode slurry based on the synergistic reinforcement of carbon nanotubes and lithium saponite, characterized in that: The positive electrode slurry includes lithium iron phosphate, binder slurry, and conductive compounding agent; the conductive compounding agent is a mixture of carbon nanotubes, conductive carbon black, and lithium saponite, and the mass ratio of carbon nanotubes, conductive carbon black, and lithium saponite is: carbon nanotubes: conductive carbon black: lithium saponite = (2.0-X):X:(0.3~0.5); X = 0.8~1.

0.

2. The highly stable lithium iron phosphate cathode slurry based on the synergistic reinforcement of carbon nanotubes and lithium saponite according to claim 1, characterized in that: The binder slurry is composed of PVDF and NMP. The mass ratio of PVDF to lithium saponite is lithium saponite:PVDF = (0.3~0.5):(1.8~2.2). The amount of NMP used is to make the solid content of the positive electrode slurry reach 65~75%.

3. The highly stable lithium iron phosphate cathode slurry based on the synergistic reinforcement of carbon nanotubes and lithium saponite according to claim 1, characterized in that: The mass ratio of lithium iron phosphate to conductive compound is lithium iron phosphate : conductive compound = (95~97) : (2.3~2.5).

4. A method for preparing a highly stable lithium iron phosphate cathode slurry based on the synergistic reinforcement of carbon nanotubes and lithium saponite, comprising the following steps: S1. The raw materials are prepared in the following mass ratio: lithium iron phosphate: carbon nanotubes: conductive carbon black: lithium saponite: PVDF = (95~97):(2.0-X):X:(0.3~0.5):(1.8~2.2). S2. Slowly add PVDF powder to NMP solvent and stir until completely dissolved to form a transparent adhesive solution; S3. The carbon nanotubes, conductive carbon black and lithium saponite are premixed and then added to the transparent adhesive solution. The mixture is dispersed at a first speed of not less than 1500 rpm to obtain a dispersion slurry. S4. Under stirring at the second rotation speed, the lithium iron phosphate is added to the dispersion slurry until the slurry is uniform; thus, a positive electrode preparation slurry is obtained. S5. The positive electrode preparation slurry is subjected to a vacuum environment and stirred at the third rotation speed to remove air bubbles, and then allowed to stand and age for more than 8 hours to obtain the high-stability lithium iron phosphate positive electrode slurry.

5. The method for preparing a highly stable lithium iron phosphate cathode slurry based on the synergistic reinforcement of carbon nanotubes and lithium saponite according to claim 4, characterized in that: In step S2, PVDF powder is slowly added to a portion of the NMP solvent, and in any of steps S3 to S5, the remaining NMP solvent is added to the system until the solid content of the positive electrode slurry reaches 65-75%.

6. The method for preparing a highly stable lithium iron phosphate cathode slurry based on the synergistic reinforcement of carbon nanotubes and lithium saponite according to claim 4, characterized in that: In step S5, the mixture is stirred at a third rotation speed not exceeding 1000 rpm under a relative vacuum of at least 0.090 MPa lower than the standard atmospheric pressure.

7. The method for preparing a highly stable lithium iron phosphate cathode slurry based on the synergistic reinforcement of carbon nanotubes and lithium saponite according to claim 4, characterized in that: In steps S2 to S5, the ambient temperature is between 20 and 30°C.

8. The method for preparing a highly stable lithium iron phosphate cathode slurry based on the synergistic reinforcement of carbon nanotubes and lithium saponite according to claim 4, characterized in that: In step S3, a dry powder mixer is used for premixing.

9. The method for preparing a highly stable lithium iron phosphate cathode slurry based on the synergistic reinforcement of carbon nanotubes and lithium saponite according to claim 4, characterized in that: In step S3, the first rotational speed is 2000~2500 rpm.

10. The method for preparing a highly stable lithium iron phosphate cathode slurry based on the synergistic reinforcement of carbon nanotubes and lithium saponite according to claim 4, characterized in that: In step S3, the second rotational speed is 1000~1500 rpm.

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