Lithium iron phosphate positive electrode slurry, preparation method thereof, positive electrode sheet and lithium battery

Abstract

This invention discloses a lithium iron phosphate (LFP) cathode slurry, its preparation method, a cathode electrode, and a lithium battery, belonging to the field of lithium-ion battery technology. The LFP cathode slurry provided by this invention includes LFP particles, thioctic acid, a conductive agent, a binder, and a solvent. By introducing thioctic acid into the LFP cathode slurry, this invention utilizes the fluidity of small thioctic acid molecules in the slurry and their adsorption on the surface of LFP particles to effectively reduce the tendency for particle aggregation and significantly improve the dispersion uniformity of the slurry. During electrode drying, thioctic acid undergoes in-situ ring-opening polymerization to form a three-dimensional polythioctic acid network structure, which possesses excellent flexibility. This structure can effectively buffer mechanical stress during electrode rolling and subsequent battery assembly, inhibiting coating cracking and peeling, thereby improving the structural stability of the electrode and ultimately improving the rate performance and cycle stability of the lithium battery.

Description

Technical Field

[0001] This invention relates to the field of lithium-ion battery technology, and in particular to a lithium iron phosphate cathode slurry and its preparation method, cathode sheet and lithium battery. Background Technology

[0002] Lithium iron phosphate (LFP) batteries are widely used in power batteries and energy storage due to their excellent cycle stability and high safety performance. However, their intrinsic electronic conductivity is relatively low. Current technologies often employ particle nano-sizing and surface carbon coating modification, but these methods have several problems: reducing particle size leads to a significant increase in the specific surface area of ​​the material, making particles prone to agglomeration and uneven dispersion during slurry preparation; carbon coating is difficult to ensure uniformity and integrity, and exposed areas are prone to side reactions with the electrolyte, and cannot suppress abnormal particle growth, resulting in discontinuous electrode conductive networks and affecting battery performance.

[0003] Existing technologies often incorporate polymer dispersants to improve slurry processing performance, but their function is limited, offering limited assistance in conductive network construction and interfacial bonding. Furthermore, their molecular chains are prone to breakage and exhibit high rigidity, easily leading to decreased dispersion efficiency and brittle coatings that are susceptible to cracking or peeling during rolling and bending. Therefore, achieving a balance between slurry dispersibility, electrode conductive network stability, and electrode structure stability is a pressing issue to be addressed in the fabrication of lithium iron phosphate batteries. Summary of the Invention

[0004] In view of this, the present invention provides a lithium iron phosphate cathode slurry and its preparation method, a cathode electrode, and a lithium battery. The present invention introduces thioctic acid into the lithium iron phosphate cathode slurry, utilizing its in-situ polymerization to form a three-dimensional network structure, significantly improving the slurry dispersibility, electrode conductivity, interfacial adhesion, and electrode flexibility, thus achieving synergistic optimization of the overall electrode performance.

[0005] In a first aspect, the present invention provides a lithium iron phosphate cathode slurry, comprising lithium iron phosphate particles, thioctic acid, a conductive agent, a binder, and a solvent; wherein the mass of the thioctic acid is 0.05% to 0.3% of the mass of the lithium iron phosphate particles.

[0006] Preferably, the mass of the thioctic acid is 0.08% to 0.25% of the mass of the lithium iron phosphate particles.

[0007] Preferably, the mass ratio of the lithium iron phosphate particles, conductive agent and binder is (90~98): (1~5): (1~5).

[0008] Preferably, the lithium iron phosphate particles are carbon-coated lithium iron phosphate particles with a carbon coating amount of 1~2wt% and a particle size D50 of 0.2~5μm.

[0009] Preferably, the conductive agent includes at least one of natural graphite, artificial graphite, conductive carbon black, carbon fiber, carbon nanotubes, graphene, conductive polymer, and metal powder; the binder includes at least one of polyvinylidene fluoride, a copolymer of polyvinylidene fluoride and hexafluoropropylene, and polytetrafluoroethylene.

[0010] Preferably, the solvent includes N At least one of methylpyrrolidone, tetrahydrofuran, dichloromethane, trichloromethane, and N,N-dimethylformamide; the viscosity of the lithium iron phosphate cathode slurry is 4000~9000 mPa·s.

[0011] Secondly, the present invention provides a method for preparing the above-mentioned lithium iron phosphate cathode slurry, comprising the following steps: The carbon-coated lithium iron phosphate particles, conductive agent, thioctic acid, and binder are mixed in a solvent to obtain the final product.

[0012] Thirdly, the present invention provides a positive electrode sheet, comprising a positive current collector and a positive active material layer disposed on at least one side surface of the positive current collector; the positive active material layer is prepared from the above-mentioned lithium iron phosphate positive electrode slurry or the lithium iron phosphate positive electrode slurry prepared by the above-mentioned preparation method.

[0013] Fourthly, the present invention provides a lithium battery comprising the above-mentioned positive electrode sheet.

[0014] Compared with the prior art, the present invention has achieved the following beneficial effects: (1) This invention introduces thioctic acid into lithium iron phosphate cathode slurry, and utilizes the fluidity of thioctic acid small molecules in the slurry and its adsorption effect on the surface of lithium iron phosphate particles to effectively reduce the tendency of particle aggregation, significantly improve the dispersion uniformity of the slurry, and achieve effective control of slurry viscosity while maintaining a high solid content.

[0015] (2) The thioctic acid described in this invention undergoes in-situ ring-opening polymerization during the electrode drying process to form a three-dimensional polythioctic acid network structure. This network structure runs through the interior of the positive electrode active material layer, which helps to build a continuous and stable conductive path and reduce the electrode resistance. At the same time, the carboxyl groups on the polythioctic acid molecular chain interact with the surface of the current collector through hydrogen bonds, which significantly improves the interfacial bonding strength between the active material layer and the current collector.

[0016] (3) The polythioctic acid three-dimensional network formed by the present invention has excellent flexibility, which can effectively buffer mechanical stress during electrode rolling and subsequent battery assembly, suppress coating cracking and peeling, thereby improving the structural stability of the electrode and ultimately improving the rate performance and cycle stability of the lithium battery. Attached Figure Description

[0017] The accompanying drawings, which form part of this specification, are used to provide a further understanding of the invention. The illustrative embodiments and descriptions of the invention are used to explain the invention and do not constitute an undue limitation thereof. Obviously, those skilled in the art can obtain other drawings based on these drawings without any inventive effort.

[0018] Figure 1 This is a schematic diagram of the thioctic acid-regulated dispersion mechanism of lithium iron phosphate cathode slurry in a specific embodiment of the present invention; Figure 2 These are the rate performance diagrams of Examples 1-3 and Comparative Examples 1-3 of the present invention. Detailed Implementation

[0019] It should be noted that the following detailed descriptions are exemplary and intended to provide further illustration of the invention. Unless otherwise specified, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains.

[0020] This invention provides a lithium iron phosphate cathode slurry, comprising lithium iron phosphate particles, thioctic acid, a conductive agent, a binder, and a solvent.

[0021] The positive electrode slurry of this invention contains lipoic acid as a multifunctional additive. Lipoic acid is a naturally occurring small molecule compound whose molecular structure contains both a disulfide five-membered ring and a carboxyl functional group. During the slurry preparation stage, lipoic acid exists in small molecule form, exhibiting good fluidity and diffusion capabilities, allowing it to be uniformly dispersed in the slurry system. Through coordination adsorption or hydrogen bonding between its carboxyl groups and metal sites or residual hydroxyl groups on the surface of lithium iron phosphate particles, it effectively anchors itself to the particle surface, particularly showing a preferential adsorption tendency for exposed areas with incomplete carbon coating. This anchoring effect generates a steric hindrance effect, transforming the interaction between particles from solid friction to liquid friction, thereby significantly reducing slurry viscosity and improving dispersion uniformity. During the subsequent electrode drying process, lipoic acid molecules undergo a ring-opening polymerization reaction of the disulfide five-membered ring upon heating, forming polylipoic acid long chains in situ, constructing a three-dimensional elastic network that runs through the entire positive electrode active material layer. A schematic diagram of the mechanism is shown below. Figure 1 As shown, this three-dimensional elastic network structure not only helps to fix the relative positions of the active particles and the conductive agent, maintaining the continuity of the conductive pathway, but also allows the carboxyl groups on its molecular chains to form strong hydrogen bonds with the oxide layer on the surface of the aluminum foil current collector, significantly improving the interfacial bonding strength between the active material layer and the current collector. Furthermore, the polythioctic acid three-dimensional network itself possesses good flexibility and mechanical stability, effectively buffering mechanical stress and inhibiting coating cracking and peeling during electrode rolling and subsequent battery assembly.

[0022] In some embodiments, the mass of the thioctic acid is 0.05% to 0.3% of the mass of the lithium iron phosphate particles. For example, 0.05%, 0.08%, 0.1%, 0.12%, 0.15%, 0.18%, 0.2%, 0.22%, 0.25%, 0.28%, or 0.3%. By limiting the mass percentage of thioctic acid to lithium iron phosphate particles, this invention can better leverage its dispersion and network-building effects. If the mass ratio is less than 0.05%, the thioctic acid content is too low, making it difficult to form an effective adsorption layer on the particle surface, resulting in insufficient steric hindrance effect, insignificant improvement in slurry dispersibility, and discontinuous polythioctic acid network formed after drying, thus limiting its effect on conductive network construction and interface bonding enhancement. If the mass ratio is greater than 0.3%, excessive thioctic acid may form an overly large three-dimensional structure during drying, hindering electron conduction, leading to increased electrode resistance, and potentially weakening the skeletal support of the binder, affecting the mechanical properties of the electrode.

[0023] More preferably, the mass of the lipoic acid is 0.08% to 0.25% of the mass of the lithium iron phosphate particles. Within this preferred range, the lipoic acid can form a suitable adsorption layer on the particle surface, achieving excellent dispersion effect. At the same time, after drying, it forms a uniform and continuous three-dimensional network structure, synergistically improving conductivity, adhesion and flexibility, resulting in optimal overall performance.

[0024] In some embodiments, the mass ratio of the lithium iron phosphate particles, conductive agent, and binder is (90~98):(1~5):(1~5). Examples include 90:5:5, 92:4:4, 93:3.5:3.5, 94:3:3, 95:2.5:2.5, 96:2:2, 97:1.5:1.5, or 98:1:1. The lithium iron phosphate particles, as the active material, provide electrochemical capacity; their content should not be too low, otherwise it will affect the battery's energy density. The conductive agent is used to construct the electron conduction network inside the electrode; if its content is too low, the conductivity will be insufficient, and if it is too high, it will reduce the proportion of active material. The binder is used to maintain the structural integrity of the active material layer; if its content is too low, the mechanical properties of the electrode will decrease, and if it is too high, it will increase resistance and reduce energy density. By using the above-mentioned ratio range, this invention can achieve a balance between conductivity and structural stability while ensuring energy density.

[0025] In some embodiments, the lithium iron phosphate particles are carbon-coated lithium iron phosphate particles with a carbon coating amount of 1-2 wt%, such as 1.0 wt%, 1.2 wt%, 1.5 wt%, 1.8 wt%, or 2.0 wt%. The carbon coating layer can improve the electronic conductivity of the lithium iron phosphate particles, but the coating process is difficult to guarantee completely uniformity, resulting in uncoated exposed areas on the particle surface. These areas are precisely the sites where lipoic acid preferentially adsorbs and anchors. Too low a carbon coating amount results in insufficient conductivity improvement, while too high a amount may affect lithium-ion diffusion. The particle size D50 is 0.2-5 μm, such as 0.2 μm, 0.5 μm, 0.8 μm, 1.0 μm, 1.5 μm, 2.0 μm, 2.5 μm, 3.0 μm, 3.5 μm, 4.0 μm, 4.5 μm, or 5.0 μm. Nanoparticles can shorten the lithium-ion diffusion path, but the increased specific surface area makes dispersion difficult. This invention can effectively solve this problem by introducing thioctic acid.

[0026] This invention does not impose any special restrictions on the preparation method of carbon-coated lithium iron phosphate particles. Regardless of whether the carbon-coated lithium iron phosphate particles are prepared using a solid-phase method, a liquid-phase method, or a gas-phase coating method, as long as there are exposed areas on their surface with incomplete carbon coating, they can adsorb the thioctic acid described in this invention, achieving the technical effects of this invention. Those skilled in the art can choose a suitable method to prepare the particles or directly use commercially available products according to actual needs.

[0027] In some embodiments, the conductive agent includes at least one selected from natural graphite, artificial graphite, conductive carbon black, carbon fiber, carbon nanotubes, graphene, conductive polymers, and metal powders. These conductive materials possess high electronic conductivity and can form a conductive network within the electrode, thereby improving electron transport efficiency. Conductive carbon black (such as Super P) is characterized by its fine particle size and large specific surface area, enabling it to fill the spaces between active particles to form a point-contact conductive network. Carbon nanotubes and graphene have one-dimensional or two-dimensional structures, enabling them to form long-range conductive networks and improve the continuity of conductive pathways. Those skilled in the art can select one or more conductive agents for use in combination according to actual needs.

[0028] In some embodiments, the binder includes at least one of polyvinylidene fluoride (PVDF), a copolymer of polyvinylidene fluoride and hexafluoropropylene (PVDF-HFP), and polytetrafluoroethylene (PTFE). These fluoropolymers possess good electrochemical stability and bonding properties, enabling them to bond active particles and conductive agents together and firmly adhere to the current collector surface. PVDF is the most commonly used binder for lithium-ion battery cathodes, exhibiting good mechanical strength and electrochemical stability. In this invention, the polythioctic acid network formed by thioctic acid synergistically works with the aforementioned binder to further enhance the structural stability of the electrode.

[0029] In some embodiments, the solvent includes at least one selected from N-methylpyrrolidone (NMP), tetrahydrofuran (THF), dichloromethane, chloroform, and N,N-dimethylformamide (DMF). These organic solvents can effectively dissolve or disperse the binder and thioctic acid to form a uniform and stable slurry system. Among them, NMP is the most commonly used solvent for PVDF binders, characterized by strong solubility and moderate volatility, making it suitable for the industrial production of cathode slurries.

[0030] In some embodiments, the viscosity of the lithium iron phosphate cathode slurry is 4000~9000 mPa·s, for example, 4000 mPa·s, 4500 mPa·s, 5000 mPa·s, 5500 mPa·s, 6000 mPa·s, 6500 mPa·s, 7000 mPa·s, 7500 mPa·s, 8000 mPa·s, 8500 mPa·s, or 9000 mPa·s. Slurry viscosity is a key parameter affecting the coating process; excessively high viscosity leads to coating difficulties and uneven coating thickness; excessively low viscosity may result in sagging and uneven edges. This invention, by introducing thioctic acid, controls the slurry viscosity within a suitable coating range while maintaining a high solids content, thus balancing processing performance and production efficiency.

[0031] In some embodiments, the solid content of the lithium iron phosphate cathode slurry is 50% to 70%, for example, 50%, 52%, 55%, 58%, 60%, 62%, 65%, 68%, or 70%. Solid content refers to the mass percentage of non-volatile components in the slurry, including lithium iron phosphate particles, conductive agents, binders, and thioctic acid. Solid content is a key parameter affecting coating efficiency, electrode areal density, and battery energy density. Too low a solid content means a high solvent content, leading to decreased coating efficiency, increased drying energy consumption, and difficulty in achieving a high areal density, thus affecting battery energy density. Too high a solid content results in excessive slurry viscosity, poor flowability, difficult coating, and problems such as uneven coating thickness and particle agglomeration. This invention introduces thioctic acid as a dispersant, utilizing its steric hindrance effect generated by adsorption on the particle surface to effectively reduce slurry viscosity while maintaining a high solid content, placing it within a suitable coating range. For example, with a solid content of 60%, the viscosity of the slurry of the present invention can be controlled between 4000 and 9000 mPa·s, exhibiting excellent coating and processing performance. Compared with traditional dispersants, the present invention does not require improving fluidity by reducing the solid content or increasing the amount of solvent, achieving excellent dispersion effects while maintaining production efficiency.

[0032] In some embodiments, the fineness of the lithium iron phosphate cathode slurry is less than 30 μm, for example, 5 μm, 8 μm, 10 μm, 12 μm, 15 μm, 18 μm, 20 μm, 22 μm, 25 μm, or 28 μm. Fineness reflects the size of particle agglomerates in the slurry; the smaller the fineness, the more uniform the dispersion. This invention, through the dispersing effect of thioctic acid, can effectively control the slurry fineness within a small range, ensuring a smooth electrode surface and uniform coating after coating.

[0033] The present invention also provides a method for preparing the above-mentioned lithium iron phosphate cathode slurry, comprising the following steps: stirring and mixing lithium iron phosphate particles, conductive agent, thioctic acid and binder in a solvent to obtain the slurry.

[0034] In some embodiments, the preparation method specifically includes the following steps: S1. Add the binder and thioctic acid to the solvent and stir to dissolve or disperse them to obtain a glue solution. The stirring time can be adjusted appropriately according to the dissolution situation, usually 2~6 hours, such as 2 hours, 3 hours, 4 hours, 5 hours or 6 hours. Taking the step of dissolving the binder and thioctic acid first is beneficial to the uniform dispersion of thioctic acid in the binder solution, creating conditions for subsequent adsorption onto the particle surface.

[0035] S2. Add the conductive agent to the adhesive solution obtained in step S1 and continue stirring and mixing for 1 to 3 hours, for example, 1 hour, 1.5 hours, 2 hours, 2.5 hours, or 3 hours. After the conductive agent is fully mixed with the adhesive solution, it can be uniformly coated by the adhesive and thioctic acid.

[0036] S3. Add the mixture obtained in step S2 to the lithium iron phosphate particles and continue stirring and mixing for 2-5 hours, for example, 2 hours, 2.5 hours, 3 hours, 3.5 hours, 4 hours, 4.5 hours, or 5 hours. During this process, the lipoic acid small molecules diffuse to the surface of the lithium iron phosphate particles due to their good fluidity, and achieve uniform anchoring through adsorption of the carboxyl groups onto the particle surface.

[0037] S4. The stirred slurry is subjected to vacuum degassing to remove air bubbles mixed in with the slurry, resulting in a uniform and stable positive electrode slurry. The degassing time is usually 0.5 to 1 hour.

[0038] The present invention, through the above-mentioned feeding sequence and stirring process, facilitates the uniform dispersion of thioctic acid in the slurry and its full adsorption on the surface of lithium iron phosphate particles, laying the foundation for in-situ polymerization in the subsequent drying process.

[0039] The present invention also provides a positive electrode sheet, comprising a positive current collector and a positive active material layer disposed on at least one side surface of the positive current collector; the positive active material layer is prepared from the positive electrode slurry.

[0040] Specifically, the positive electrode sheet can be prepared by uniformly coating the above-mentioned positive electrode slurry onto at least one side of the positive electrode current collector, followed by drying and rolling. The coating method can be conventional methods in the art, such as transfer coating or extrusion coating. The drying temperature is typically 80℃~150℃, for example, 80℃, 90℃, 100℃, 110℃, 120℃, 130℃, 140℃, or 150℃. During this drying process, the solvent gradually evaporates, and simultaneously, thioctic acid undergoes a ring-opening polymerization reaction upon heating, forming a three-dimensional polythioctic acid network that penetrates between the active particles and the conductive agent, effectively connecting the components and tightly bonding to the current collector surface through hydrogen bonds. The dried electrode sheet is then compacted to the target thickness using a roller press to obtain the final positive electrode sheet. The positive electrode current collector can be aluminum foil or aluminum foil with a conductive coating on its surface, typically with a thickness of 8~20 μm.

[0041] The present invention also provides a lithium battery, including the above-mentioned positive electrode, and further including a negative electrode, a separator, an electrolyte and a battery casing, wherein the separator is disposed between the positive electrode and the negative electrode.

[0042] The lithium battery of the present invention can be a conventional form in the art, such as a square stacked battery, a pouch stacked battery, a square wound battery, or a cylindrical wound battery.

[0043] In some embodiments, the negative electrode sheet includes a negative electrode current collector and a layer of negative electrode active material disposed on at least one side of the negative electrode current collector. The negative electrode active material can be selected from conventional negative electrode materials in the art, such as artificial graphite, natural graphite, mesophase carbon microspheres, silicon-carbon composite materials, and silicon-oxygen composite materials. The negative electrode current collector can be copper foil or copper foil with a conductive coating on its surface.

[0044] In some embodiments, the separator comprises a polyolefin porous base membrane, such as a polyethylene (PE) membrane, a polypropylene (PP) membrane, or a PP / PE composite membrane. At least one surface of the separator may be provided with a heat-resistant coating (such as a ceramic coating) and / or an adhesive coating (such as a PVDF coating) to improve the thermal stability of the separator and its adhesion to the electrode.

[0045] In some embodiments, the electrolyte includes an electrolyte salt, an organic solvent, and additives. The electrolyte salt may be lithium hexafluorophosphate (LiPF6), lithium tetrafluoroborate (LiBF4), lithium bis(oxalato)borate (LiBOB), etc.; the organic solvent may be at least one of ethylene carbonate (EC), dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), diethyl carbonate (DEC), etc.; and the additives may be vinylene carbonate (VC), fluoroethylene carbonate (FEC), 1,3-propanesulfonic acid lactone (PS), etc.

[0046] The lithium battery of the present invention uses the above-mentioned positive electrode sheet, in which a three-dimensional polythiooctanoic acid network structure is formed in the positive electrode active material layer. The slurry is uniformly dispersed, the electrode sheet has good conductivity, strong interfacial bonding force and high structural stability, thus exhibiting excellent rate performance and cycle stability.

[0047] The technical solution of the present invention will be further described below with reference to specific embodiments. The present invention does not impose any special restrictions on the source of reagents used in the following embodiments; commercially available products well known to those skilled in the art can be used.

[0048] Example 1 This embodiment provides a method for preparing lithium iron phosphate cathode slurry and its cathode sheet, specifically including the following steps: 1. Preparation of positive electrode slurry (1) Add the adhesive polyvinylidene fluoride (PVDF) and thioctic acid (TA) to the solvent N-methylpyrrolidone (NMP), and place them in a high-speed homogenizer to stir and dissolve and disperse them to obtain a glue solution. The mass of thioctic acid is 0.1% of the mass of the lithium iron phosphate particles added later. The stirring time is 4 hours to ensure that the adhesive is completely dissolved and the thioctic acid is uniformly dispersed in the glue solution.

[0049] (2) Add the conductive agent Super P to the adhesive solution obtained in step (1) and continue stirring and mixing for 2 hours. After the conductive agent and adhesive solution are fully mixed, they are uniformly coated by the adhesive and thioctic acid.

[0050] (3) The mixture obtained in step (2) is added to carbon-coated lithium iron phosphate particles (carbon coating amount of 1.5 wt%, particle size D50 of 1.2 μm), and the mixture is stirred for 3 hours. During this process, the thioctic acid molecules diffuse to the surface of the lithium iron phosphate particles due to their good fluidity, and achieve uniform anchoring through adsorption of the carboxyl groups on the particle surface. The mass ratio of lithium iron phosphate particles, conductive agent and binder is 95:2.5:2.5, and the solid content of the slurry is controlled at 60%.

[0051] (4) Vacuum degassing: The slurry obtained in step (3) is subjected to vacuum degassing to remove the air bubbles mixed in the slurry. The degassing time is 0.5 hours to obtain a uniform and stable positive electrode slurry.

[0052] 2. Preparation of the positive electrode sheet The above-mentioned positive electrode slurry was uniformly coated onto the surface of the aluminum foil current collector using a transfer coating method, with a coating surface density of 220 g / m². 2 The aluminum foil current collector thickness is 12 μm. The coated electrode is placed in a vacuum drying oven at 120℃ and dried for 12 hours. The dried electrode is then removed and compacted on a roller press to a compaction density of 2.6 g / cc, yielding the final positive electrode.

[0053] Example 2 This embodiment provides a method for preparing lithium iron phosphate cathode slurry and its cathode sheet, specifically including the following steps: 1. Preparation of positive electrode slurry (1) Add the adhesive polyvinylidene fluoride (PVDF) and thioctic acid (TA) to the solvent N-methylpyrrolidone (NMP), and place them in a high-speed homogenizer for stirring to dissolve and disperse, thereby obtaining a glue solution. The mass of thioctic acid is 0.2% of the mass of the lithium iron phosphate particles added later. The stirring time is 4 hours to ensure that the adhesive is completely dissolved and the thioctic acid is uniformly dispersed in the glue solution.

[0054] (2) Add the conductive agent Super P to the adhesive solution obtained in step (1) and continue stirring and mixing for 2 hours. After the conductive agent and adhesive solution are fully mixed, they are uniformly coated by the adhesive and thioctic acid.

[0055] (3) The mixture obtained in step (2) is added to carbon-coated lithium iron phosphate particles (carbon coating amount of 1.5 wt%, particle size D50 of 1.2 μm), and the mixture is stirred for 3 hours. During this process, the thioctic acid molecules diffuse to the surface of the lithium iron phosphate particles due to their good fluidity, and achieve uniform anchoring through adsorption of the carboxyl groups on the particle surface. The mass ratio of lithium iron phosphate particles, conductive agent and binder is 95:2.5:2.5, and the solid content of the slurry is controlled at 60%.

[0056] (4) Vacuum degassing: The slurry obtained in step (3) is subjected to vacuum degassing to remove the air bubbles mixed in the slurry. The degassing time is 0.5 hours to obtain a uniform and stable positive electrode slurry.

[0057] 2. Preparation of the positive electrode sheet The above-mentioned positive electrode slurry was uniformly coated onto the surface of the aluminum foil current collector using a transfer coating method, with a coating surface density of 220 g / m². 2 The aluminum foil current collector thickness is 12 μm. The coated electrode is placed in a vacuum drying oven at 120℃ and dried for 12 hours. The dried electrode is then removed and compacted on a roller press to a compaction density of 2.6 g / cc, yielding the final positive electrode.

[0058] Example 3 This embodiment provides a method for preparing lithium iron phosphate cathode slurry and its cathode sheet, specifically including the following steps: 1. Preparation of positive electrode slurry (1) Add the adhesive polyvinylidene fluoride (PVDF) and thioctic acid (TA) to the solvent N-methylpyrrolidone (NMP), and place them in a high-speed homogenizer for stirring to dissolve and disperse, thereby obtaining a glue solution. The mass of thioctic acid is 0.3% of the mass of the lithium iron phosphate particles added later. The stirring time is 4 hours to ensure that the adhesive is completely dissolved and the thioctic acid is uniformly dispersed in the glue solution.

[0059] (2) Add the conductive agent Super P to the adhesive solution obtained in step (1) and continue stirring and mixing for 2 hours. After the conductive agent and adhesive solution are fully mixed, they are uniformly coated by the adhesive and thioctic acid.

[0060] (3) The mixture obtained in step (2) is added to carbon-coated lithium iron phosphate particles (carbon coating amount of 1.5 wt%, particle size D50 of 1.2 μm), and the mixture is stirred for 3 hours. During this process, the thioctic acid molecules diffuse to the surface of the lithium iron phosphate particles due to their good fluidity, and achieve uniform anchoring through adsorption of the carboxyl groups on the particle surface. The mass ratio of lithium iron phosphate particles, conductive agent and binder is 95:2.5:2.5, and the solid content of the slurry is controlled at 60%.

[0061] (4) Vacuum degassing: The slurry obtained in step (3) is subjected to vacuum degassing to remove the air bubbles mixed in the slurry. The degassing time is 0.5 hours to obtain a uniform and stable positive electrode slurry.

[0062] 2. Preparation of the positive electrode sheet The above-mentioned positive electrode slurry was uniformly coated onto the surface of the aluminum foil current collector using a transfer coating method, with a coating surface density of 220 g / m². 2 The aluminum foil current collector thickness is 12 μm. The coated electrode is placed in a vacuum drying oven at 120℃ and dried for 12 hours. The dried electrode is then removed and compacted on a roller press to a compaction density of 2.6 g / cc, yielding the final positive electrode.

[0063] Comparative Example 1 The difference between this comparative example and Example 3 is that thioctic acid was replaced with polyethylene glycol octylphenyl ether (Triton X-100) dispersant.

[0064] Comparative Example 2 The difference between this comparative example and Example 3 is that thioctic acid was replaced with polyvinylpyrrolidone (PVP) dispersant.

[0065] Comparative Example 3 The difference between this comparative example and Example 3 is that hydrogenated nitrile butyrate (HNBR) dispersant is used instead of thioctic acid.

[0066] Comparative Example 4 The difference between this comparative example and Example 3 is that the mass of thioctic acid in step (1) is 1.0% of the mass of lithium iron phosphate particles (excessive addition).

[0067] Test case 1. Positive electrode performance testing The following tests were performed on the positive electrode sheets of Examples 1-3 and Comparative Examples 1-4: (1) Slurry viscosity test The discharge viscosity of the cathode slurry obtained in each embodiment and comparative example was tested using a rotational viscometer (Brookfield DV2T) at 25°C. A suitable rotor was selected, and the rotational speed was set to 20 rpm. Viscosity values ​​were recorded after the readings stabilized, and the unit was mPa·s. Each sample was tested three times, and the average value was taken.

[0068] (2) Compacted density test The compaction density of the electrode is calculated based on the coating surface density, the thickness of the electrode after roll forming, and the thickness of the active material layer, with units of g / cc. The formula for calculating compaction density is: Compaction density = Coating surface density / Active material layer thickness.

[0069] (2) Electrode resistivity test A four-probe electrode resistance testing system was used to test the rolled electrodes at room temperature. Five different positions were selected along the longitudinal and transverse directions for each sample, and the average value was taken as the electrode resistivity, in Ω.

[0070] (3) Peel force test The electrode was cut into 20mm wide strips and subjected to a 180° peel test using a universal testing machine. With the active material layer facing down, the electrode was fixed to the test platform with double-sided tape. The active material layer was peeled from the aluminum foil current collector at a constant speed of 50mm / min. The force values ​​during the peeling process were recorded, and the average peel force was calculated in N / m. Each sample was tested three times, and the average value was taken.

[0071] (4) Test of the number of times light is transmitted after folding Cut the electrode sheet into rectangular samples of 50mm × 20mm, fold it repeatedly in the same direction, and observe whether a light-transmitting crack appears at the crease after each fold. Record the number of folds when the electrode sheet first shows a clear light-transmitting crack. Test each sample 3 times and take the average value.

[0072] The performance test results of the positive electrode slurry and positive electrode sheet of each embodiment and comparative example are shown in Table 1.

[0073] Table 1 Performance test results of positive electrode slurry and positive electrode sheet sample Viscosity (mPa·s) Compacted density (g / cc) Electrode resistivity (Ω) Peel force (N / m) Number of times light passes through when folded Example 1 7900 2.6 0.511 25 4 Example 2 6600 2.6 0.482 29 5 Example 3 5100 2.6 0.451 31 5 Comparative Example 1 9450 2.45 0.562 20 2 Comparative Example 2 8320 2.45 0.523 25 3 Comparative Example 3 8510 2.45 0.541 21 3 Comparative Example 4 8900 2.46 0.624 22 3 As shown in Table 1, under the same solid content (60%), the slurry viscosities (5100~7900 mPa·s) of Examples 1-3 were significantly lower than those of Comparative Examples 1-3 (8320~9450 mPa·s). This indicates that thioctic acid, as a small molecule dispersant, can effectively adsorb onto the surface of lithium iron phosphate particles, inhibiting particle agglomeration and improving slurry flowability through steric hindrance. Specifically, as the amount of thioctic acid increased from 0.1% to 0.3%, the slurry viscosity showed a decreasing trend, indicating that increasing the amount of TA within the preferred range is beneficial for improving the dispersion effect. The viscosity of Comparative Example 4 (with excessive TA addition) was 8900 mPa·s, higher than that of Example 3, indicating that when the amount of thioctic acid is too high, excessive TA may form local crosslinking or phase separation in the slurry, thus affecting the dispersion effect.

[0074] The electrode resistivity of Examples 1-3 (0.451-0.511 Ω) was lower than that of Comparative Examples 1-3 (0.523-0.562 Ω), and a lower resistivity was maintained while achieving a higher compaction density (2.5-2.6 g / cc). This indicates that the polythiooctanoic acid three-dimensional network formed by the in-situ polymerization of TA during the drying process effectively connects the active particles and the conductive agent, constructing a continuous and stable conductive pathway. The electrode resistivity of Comparative Example 4 (0.624 Ω) was significantly higher than that of the Examples, indicating that the excessively thick polymer layer formed by excess TA actually hindered electron conduction.

[0075] The peel strength of Examples 1-3 (25-31 N / m) was generally higher than that of Comparative Examples 1-3 (20-25 N / m), indicating that the carboxyl groups on the polythiooctanoic acid molecular chain formed strong hydrogen bonds with the oxide layer on the surface of the aluminum foil current collector, significantly improving the interfacial bonding strength between the active material layer and the current collector. The peel strength of Comparative Example 4 (22 N / m) was lower than that of the Examples, which may be related to the instability of the network structure formed by the excess TA.

[0076] In the folding and light transmission test, Examples 1-3 required 4-5 folds before cracking and light transmission occurred, while Comparative Examples 1-3 cracked after only 2-3 folds. This indicates that the polythioctic acid (TA) three-dimensional network has good flexibility and can effectively buffer the mechanical stress generated during bending, inhibiting coating cracking. Comparative Example 4 required 3 folds, fewer than the Examples, suggesting that excessive TA may have compromised the mechanical stability of the network.

[0077] 2. Battery performance test In an argon-filled glove box, CR2025 coin cell half-cells were assembled in the following order: positive electrode shell, positive electrode plate, separator, electrolyte, lithium plate, gasket, spring contact, and negative electrode shell. The charge / discharge voltage range was 2.0-3.8V, and the nominal specific capacity was 1C = 170mAh / g. The following electrochemical tests were performed: (1) Rate performance test: Charge and discharge tests were performed at 0.1C, 0.5C, 1.0C, 2.0C and 0.1C respectively. Each rate was cycled 5 times. The discharge specific capacity at each rate was recorded, and the ratio of the discharge specific capacity at 2C rate to the discharge specific capacity at the initial 0.1C rate (2C / 0.1C capacity retention rate) was calculated.

[0078] (2) Cyclic stability test: 100 charge-discharge cycles were performed at 1C rate. The discharge specific capacity of the first cycle and the 100th cycle was recorded, and the capacity retention rate after 100 cycles was calculated.

[0079] The test results are summarized in Table 2 and Figure 2 .

[0080] Table 2 Electrochemical performance test results of button batteries sample 0.1C discharge specific capacity (mAh / g) 2C discharge specific capacity (mAh / g) 2C / 0.1C Capacity Retention Rate (%) 100-cycle capacity retention (%) Example 1 158.9 141.8 89.3 94.2 Example 2 159.0 142.8 89.8 95.6 Example 3 159.2 143.3 90.0 96.1 Comparative Example 1 152.4 132.3 86.8 88.3 Comparative Example 2 153.3 133.2 86.9 90.5 Comparative Example 3 152.8 132.7 86.8 89.2 As can be seen from the test results in Table 2, the discharge specific capacity (158~160mAh / g) of the button batteries assembled in Examples 1~3 at a 0.1C rate is slightly higher than that of Comparative Examples 1~3 (152~154mAh / g), indicating that the introduction of thioctic acid did not have an adverse effect on the intrinsic capacity of the active material.

[0081] In terms of rate performance, the 2C discharge specific capacity (140-144 mAh / g) of Examples 1-3 was significantly higher than that of Comparative Examples 1-3 (132-134 mAh / g), and the 2C / 0.1C capacity retention was also better than that of the comparative examples. This indicates that the three-dimensional polythioctic acid network formed by in-situ polymerization of TA constructs a continuous conductive pathway, reduces electrode polarization, and allows the active material to fully utilize its capacity under high current discharge conditions.

[0082] Regarding cycle stability, the capacity retention rates of Examples 1-3 after 100 cycles were all higher than those of Comparative Examples 1-3. This is attributed to the multifaceted synergistic effects of TA: uniform dispersion ensures that the active material fully participates in the reaction during cycling; the strong hydrogen bonding between the polythioctic acid network and the current collector maintains the stability of the interfacial bonding; and the flexibility of the three-dimensional network effectively buffers the volume change stress during cycling and inhibits the destruction of the electrode structure.

[0083] In summary, this invention introduces an appropriate amount of thioctic acid into the lithium iron phosphate cathode slurry and utilizes its in-situ polymerization to form a three-dimensional polythioctic acid network, thereby achieving a synergistic improvement in slurry dispersibility, electrode conductivity, interfacial bonding force, and electrode flexibility, ultimately significantly improving the rate performance and cycle stability of lithium batteries.

[0084] The above description is merely a preferred embodiment of the present invention and is not intended to limit the invention. Various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A lithium iron phosphate cathode slurry, characterized in that, It includes lithium iron phosphate particles, thioctic acid, a conductive agent, a binder, and a solvent; the mass of the thioctic acid is 0.05% to 0.3% of the mass of the lithium iron phosphate particles.

2. The lithium iron phosphate cathode slurry as described in claim 1, characterized in that, The mass of the thioctic acid is 0.08% to 0.25% of the mass of the lithium iron phosphate particles.

3. The lithium iron phosphate cathode slurry as described in claim 2, characterized in that, The mass ratio of the lithium iron phosphate particles, conductive agent and binder is (90~98): (1~5): (1~5).

4. The lithium iron phosphate cathode slurry as described in claim 1, characterized in that, The lithium iron phosphate particles are carbon-coated lithium iron phosphate particles with a carbon coating amount of 1~2wt% and a particle size D50 of 0.2~5μm.

5. The lithium iron phosphate cathode slurry as described in claim 1, characterized in that, The conductive agent includes at least one of natural graphite, artificial graphite, conductive carbon black, carbon fiber, carbon nanotubes, graphene, conductive polymers, and metal powder.

6. The lithium iron phosphate cathode slurry as described in claim 1, characterized in that, The adhesive includes at least one of polyvinylidene fluoride, a copolymer of polyvinylidene fluoride and hexafluoropropylene, and polytetrafluoroethylene.

7. The lithium iron phosphate cathode slurry as described in claim 1, characterized in that, The solvent includes N At least one of methylpyrrolidone, tetrahydrofuran, dichloromethane, trichloromethane, and N,N-dimethylformamide; the viscosity of the lithium iron phosphate cathode slurry is 4000~9000 mPa·s.

8. The method for preparing lithium iron phosphate cathode slurry according to any one of claims 1 to 7, characterized in that, Includes the following steps: The carbon-coated lithium iron phosphate particles, conductive agent, thioctic acid, and binder are mixed in a solvent to obtain the final product.

9. A positive electrode sheet, characterized in that, It includes a positive current collector and a positive active material layer disposed on at least one side surface of the positive current collector; the positive active material layer is prepared from the lithium iron phosphate positive electrode slurry according to any one of claims 1 to 7 or the lithium iron phosphate positive electrode slurry prepared by the preparation method according to claim 8.

10. A lithium battery, characterized in that, Includes the positive electrode sheet as described in claim 9.

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