Lithium manganese iron phosphate positive electrode material slurry, preparation method thereof and lithium ion battery

Abstract

The invention provides lithium manganese iron phosphate positive electrode material slurry, a preparation method thereof and a lithium ion battery. The preparation method comprises the following steps: preparing materials; mixing: adding the binder into a part of the solvent, and carrying out first mixing to obtain first slurry; a conductive agent is added into the first slurry, and second slurry is obtained through second mixing; the lithium manganese iron phosphate positive electrode material is divided into a positive electrode material part A and a positive electrode material part B; adding the part A of the positive electrode material into the second slurry, and performing third mixing to obtain third slurry; adding the part B of the positive electrode material into the third slurry, and performing fourth mixing to obtain fourth slurry; adding a dispersing agent into the fourth slurry, and performing fifth mixing to obtain fifth slurry; adding the other part of the solvent into the fifth slurry, and performing sixth mixing to obtain mixed slurry; and post-processing. By optimizing the mixing sequence, dispersion and combination of all components in the slurry are enhanced, the stability of the obtained slurry is further improved, and then the electrochemical performance of the lithium ion battery obtained by coating the slurry is improved.

Description

Technical Field

[0001] The present invention relates to the field of lithium ion batteries, and in particular to a lithium iron manganese phosphate positive electrode material slurry, a preparation method thereof and a lithium ion battery. Background Art

[0002] As an "upgraded version" of lithium iron phosphate, lithium manganese iron phosphate inherits the advantages of LFP such as low cost, high thermal stability, and high safety, while making up for the shortcomings of lithium iron phosphate such as low energy density and poor low-temperature stability. However, the positive electrode material slurry prepared by lithium manganese iron phosphate is prone to gelation, which is manifested as high slurry viscosity and poor dispersion of lithium iron phosphate particles, which makes the positive electrode slurry coating process difficult to carry out. At the same time, the fineness of the slurry is high after storage, and the coating surface density fluctuates. The slurry defects in the above processing process will affect the capacity consistency of the battery and the N / P design of the pole piece, resulting in local lithium precipitation of the prepared battery at the end of the cycle, and the growth of lithium dendrites piercing the diaphragm, causing serious safety hazards. At the same time, it also makes the mass production of lithium-ion batteries with lithium manganese iron phosphate as the positive electrode material into a bottleneck.

[0003] At present, the conventional method to solve the poor stability of lithium manganese iron phosphate slurry and poor conductivity of the pole piece is mainly to dope nickel cobalt manganese oxide ternary materials, or to specially design the positive electrode material itself. In addition, during the production and processing, the solid content in the slurry is reduced to ensure the viscosity is qualified. However, the above methods are either too complicated, or reduce production efficiency, and greatly increase production costs.

[0004] Based on this, how to start from the formula of the positive electrode material slurry and adjust and optimize its mixed slurry preparation method to obtain a lithium manganese iron phosphate positive electrode material slurry with higher stability and better cycle stability of the lithium-ion battery after coating is one of the important technical problems that need to be solved in this field. Summary of the invention

[0005] The main purpose of the present invention is to provide a lithium iron manganese phosphate positive electrode material slurry, a preparation method thereof and a lithium ion battery, so as to solve the problems in the prior art that the lithium iron manganese phosphate positive electrode slurry has poor stability, easy gelation, poor conductivity, and the resulting poor battery capacity consistency and poor cycle performance.

[0006] In order to achieve the above-mentioned purpose, the first aspect of the present invention provides a method for preparing a lithium iron manganese phosphate positive electrode material slurry, comprising: step S1, ingredients: by weight, the lithium iron manganese phosphate positive electrode material slurry comprises 94 to 98 parts of lithium iron manganese phosphate positive electrode material, 1 to 3 parts of a conductive agent, 1 to 2.5 parts of a binder, 0.1 to 0.5 parts of a dispersant, and the balance is a solvent; step S2, mixing: step S2-1, adding a binder to a portion of the solvent, and obtaining a first slurry through a first mixing; step S2-2, adding a conductive agent to the first slurry, and obtaining a second slurry through a second mixing; step S2-3, dividing the lithium iron manganese phosphate positive electrode material into a positive electrode material part A and a positive electrode material part B. Material B part; adding positive electrode material part A to the second slurry, and obtaining a third slurry through a third mixing; adding positive electrode material part B to the third slurry, and obtaining a fourth slurry through a fourth mixing; step S2-4, adding a dispersant to the fourth slurry, and obtaining a fifth slurry through a fifth mixing; step S2-5, adding another part of the solvent to the fifth slurry, and obtaining a mixed slurry through a sixth mixing; the weight ratio of the binder to a part of the solvent is 1: (19-20); the weight ratio of the positive electrode material part A to the positive electrode material part B is 1: (1.0-1.2); step S3, post-treatment: the mixed slurry is successively subjected to vacuum defoaming treatment and filtration treatment to obtain lithium manganese iron phosphate positive electrode material slurry.

[0007] Further, the conductive agent includes a first carbon material and a second carbon material; the first carbon material is a zero-dimensional carbon material, and the second carbon material is a one-dimensional carbon material and / or a two-dimensional carbon material; preferably, the zero-dimensional carbon material is conductive carbon black and / or conductive graphite; the one-dimensional carbon material is selected from one or more of carbon fibers, single-walled carbon nanotubes and multi-walled carbon nanotubes; the two-dimensional carbon material is graphene; more preferably, the solvent is nitrogen-methylpyrrolidone.

[0008] Furthermore, the weight ratio of the first carbon material to the second carbon material is (0.01-4.5):1; preferably, the conductive agent includes a zero-dimensional carbon material and a one-dimensional carbon material, and the weight ratio of the zero-dimensional carbon material to the one-dimensional carbon material is (1-3.5):1.

[0009] Furthermore, step S2-2 includes: adding the first carbon material to the first slurry, mixing it in a stage with an orbital speed of 10rpm to 25rpm and a time of 10min to 1h to obtain a first mixed slurry; adding the second carbon material to the first mixed slurry, mixing it in a second stage with an orbital speed of 10rpm to 25rpm and a rotation speed of 1800rpm to 1900rpm to obtain a second slurry.

[0010] Further, the dispersant is selected from at least two of a polycyclic dispersant, a phosphoric acid dispersant, an aliphatic hydrocarbon dispersant, a polyether dispersant and an amide dispersant; preferably, the polycyclic dispersant is cyclodextrin; and / or, the phosphoric acid dispersant is triethyl phosphate and / or triphenyl phosphate; and / or, the aliphatic hydrocarbon dispersant is selected from one or more of n-hexadecane, isohexadecane and n-dodecane; and / or, the polyether dispersant is selected from one or more of polyethylene glycol, polypropylene glycol and polytetrahydrofuran; and / or, the amide dispersant is selected from one or more of polyacrylamide, N,N-dimethylformamide and N-vinylpyrrolidone; more preferably, the dispersant is selected from at least four of a polycyclic dispersant, a phosphoric acid dispersant, an aliphatic hydrocarbon dispersant, a polyether dispersant and an amide dispersant, and the dispersant includes a polycyclic dispersant, a polyether dispersant and an amide dispersant.

[0011] Further, in step S2, the first mixing, the second mixing, the third mixing, the fourth mixing, the fifth mixing and the sixth mixing are all carried out by a double planetary mixer, and: the revolution speeds of the first mixing, the fifth mixing and the sixth mixing are each independently 20rpm~40rpm, preferably 20rpm~25rpm, and the rotation speeds are each independently 1000rpm~2000rpm, preferably 1900rpm~2000rpm; preferably, the time for the first mixing is 2.5h~3.0h; and / or, the time for the fifth mixing is 3.5h~4.0h; and / or, the time for the sixth mixing is 0.5h~1.0h.

[0012] Further, step S2-3 includes: adding part of the positive electrode material A to the second slurry, and obtaining a third slurry through a third mixing process with an orbital speed of 10rpm to 15rpm and a time of 10min to 15min; adding part of the positive electrode material B to the third slurry, and obtaining a fourth slurry through a fourth mixing process with an orbital speed of 10rpm to 15rpm and a time of 0.5h to 1h.

[0013] The second aspect of the present invention provides a lithium iron manganese phosphate positive electrode material slurry, which is prepared by the above-mentioned method for preparing the lithium iron manganese phosphate positive electrode material slurry.

[0014] Further, at 25±2° C., the viscosity of the lithium manganese iron phosphate positive electrode material slurry is recorded as V1; the viscosity of the lithium manganese iron phosphate positive electrode material slurry after standing for 24 hours is recorded as V2; (V2-V1) / V1×100%=78%~160%.

[0015] The third aspect of the present invention provides a lithium-ion battery, comprising a positive electrode sheet, wherein the positive electrode sheet comprises a positive electrode collector and a positive electrode active layer arranged on the surface of at least one side of the positive electrode collector, wherein the positive electrode active layer is obtained by coating, drying and hot pressing the above-mentioned lithium manganese iron phosphate positive electrode material slurry in sequence.

[0016] By applying the technical solution of the present invention, the stability of the lithium manganese iron phosphate positive electrode material slurry is effectively improved through fine formula control and mixing step design. The method of adding the positive electrode material in stages can better control the viscosity and fluidity of the slurry, which is conducive to improving the uniformity and consistency of the coating. By optimizing the mixing sequence, the dispersion and combination of the components in the slurry are further enhanced, the stability of the obtained slurry is further improved, and then the electrochemical performance of the lithium ion battery obtained by the coating is improved. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] The drawings constituting a part of the present application are used to provide a further understanding of the present invention. The exemplary embodiments of the present invention and their descriptions are used to explain the present invention and do not constitute an improper limitation of the present invention. In the drawings:

[0018] Figure 1 The viscosity of the slurries obtained in the embodiments of the present invention and the comparative examples varies with the standing time;

[0019] Figure 2 The graphs are the variation curves of the solid content of the upper and lower layers of the slurries obtained in the embodiments of the present invention and the comparative examples as a function of the standing time. DETAILED DESCRIPTION

[0020] It should be noted that, in the absence of conflict, the embodiments and features in the embodiments of the present application can be combined with each other. The present invention will be described in detail below in conjunction with the embodiments.

[0021] As described in the background technology, the lithium iron manganese phosphate positive electrode slurry in the prior art has poor stability, is easy to gel, and has poor conductivity, which leads to poor capacity consistency and poor cycle performance of lithium-ion batteries. In order to solve the above technical problems, the first aspect of the present invention provides a method for preparing a lithium iron manganese phosphate positive electrode material slurry, comprising: step S1, ingredients: by weight, the lithium iron manganese phosphate positive electrode material slurry includes 94 to 98 parts of lithium iron manganese phosphate positive electrode material, 1 to 3 parts of conductive agent, 1 to 2.5 parts of binder, 0.1 to 0.5 parts of dispersant, and the balance is solvent; step S2, mixing: step S2-1, adding the binder to a part of the solvent, after a first mixing, to obtain a first slurry; step S2-2, adding the conductive agent to the first slurry, after a second mixing, to obtain a second slurry; step S2-3, dividing the lithium iron manganese phosphate positive electrode material into a positive electrode material part A and a positive The cathode material B part; adding the cathode material A part to the second slurry, and mixing for the third time to obtain the third slurry; adding the cathode material B part to the third slurry, and mixing for the fourth time to obtain the fourth slurry; step S2-4, adding the dispersant to the fourth slurry, and mixing for the fifth time to obtain the fifth slurry; step S2-5, adding another part of the solvent to the fifth slurry, and mixing for the sixth time to obtain a mixed slurry; the weight ratio of the binder to a part of the solvent is 1: (19-20); the weight ratio of the cathode material A part to the cathode material B part is 1: (1.0-1.2); step S3, post-treatment: the mixed slurry is successively subjected to vacuum defoaming treatment and filtration treatment to obtain lithium manganese iron phosphate cathode material slurry.

[0022] The above-mentioned preparation method provided by the present invention effectively improves the stability, conductivity and coating performance of the obtained lithium iron manganese phosphate positive electrode slurry through a carefully designed proportion of ingredients and a staged mixing step, thereby improving the cycle performance of the final lithium battery. Specifically: First, the stability of the positive electrode material slurry is mainly affected by the electrostatic repulsion between particles, the van der Waals force and the wet effect of the solvent. However, due to the particularity of the positive electrode material, the traditional lithium iron manganese phosphate slurry is easy to form agglomerates during the stirring process, resulting in an increase in the viscosity of the slurry and a decrease in stability. The present invention enhances the dispersibility and stability between particles by controlling the ratio of lithium iron manganese phosphate to the conductive agent, the binder and the dispersant as above. In particular, the addition of the above-mentioned specific weight of the conductive agent and the dispersant not only provides additional charge dispersion, but also improves the suspension state of the particles in the solvent through its surface activity. On this basis, it is more important to add the above-mentioned component materials in stages according to the above-mentioned order and fully mix them to form a uniform conductive network, thereby effectively preventing the sedimentation and gelation of the obtained positive electrode material slurry. In particular, the present invention adds lithium manganese iron phosphate material in stages, first preliminarily mixes a part of the material with the slurry, and then adds the remaining material for in-depth mixing. This process reduces large agglomerations between particles, improves the dispersibility of the material, and further optimizes the viscosity, fineness and stability of the slurry. Also, in practical applications, the weight ratio of the positive electrode material part A to the positive electrode material part B can be (1:1), (1:1.05), (1:1.1), (1:1.15), (1:1.2), and the numerical range formed between any two points. At the same time, by adding a dispersant and adjusting the solvent ratio in the final stage, its stability is further improved, so that it still maintains good fluidity at a high solid content, which is conducive to uniform coating during the subsequent preparation of the battery, avoiding local lithium precipitation and the growth of lithium dendrites, and ultimately improving the safety, consistency and long-cycle stability of the resulting lithium-ion battery.

[0023] Further, the conductive agent includes a first carbon material and a second carbon material; the first carbon material is a zero-dimensional carbon material, and the second carbon material is a one-dimensional carbon material and / or a two-dimensional carbon material. In the preparation method provided by the present invention, the conductive agent includes a zero-dimensional carbon material and a one-dimensional carbon material and / or a two-dimensional carbon material, wherein the zero-dimensional carbon material can be evenly distributed on the surface of the positive electrode material particles due to its high specific surface area and good electrical conductivity, forming point contact, providing anchor points for the one-dimensional and two-dimensional carbon materials, thereby building a basic conductive network in the positive electrode slurry. The one-dimensional carbon material and / or the two-dimensional carbon material can be used as a conductive plane to provide a fast path for a large number of electrons to be transmitted. When used in conjunction with the zero-dimensional carbon material, a more complex, efficient and stable multi-dimensional conductive network can be constructed, significantly improving the conductivity of the positive electrode material. In several typical embodiments, the zero-dimensional carbon material is conductive carbon black and / or conductive graphite; the one-dimensional carbon material is selected from one or more of carbon fiber, single-walled carbon nanotube and multi-walled carbon nanotube; the two-dimensional carbon material is graphene.

[0024] In order to further improve the stability and fluidity of the obtained slurry, the preferred solvent is nitrogen-methyl pyrrolidone, which is a polar aprotic solvent with lower toxicity than other organic solvents. It can also promote the dissolution of the binder and significantly improve the dispersion between the positive electrode active material and the binder, thereby forming a more uniform and stable slurry.

[0025] Furthermore, the weight ratio of the first carbon material to the second carbon material is (0.01-4.5):1. At this ratio, the first carbon material and the second carbon material can work synergistically to build a more continuous and multi-dimensional conductive network, thereby significantly reducing the contact resistance between the active materials and improving the conductivity of the positive electrode slurry after coating. At the same time, the conductive network formed by this weight ratio can also improve the fluidity of the slurry through its structural characteristics, inhibit its gelation and agglomeration, and prolong the stability of the slurry, which is conducive to obtaining a lithium-ion battery with superior electrical properties after subsequent coating. Preferably, the conductive agent includes a zero-dimensional carbon material and a one-dimensional carbon material, and the weight ratio of the zero-dimensional carbon material to the one-dimensional carbon material is (1-3.5):1. Among the second carbon materials, especially the one-dimensional carbon material, it has a long radial structure, which can form a conductive channel through the positive electrode active layer obtained by slurry coating, forming a bridging effect. When it is mixed with zero-dimensional carbon material in the above-mentioned dosage ratio, it can more significantly increase the electron transmission path, thereby further improving the continuity and conductivity of the formed conductive network, and then improving the electrical performance of the lithium-ion battery obtained after coating.

[0026] In several more typical embodiments, the conductive agent includes conductive carbon black, carbon fiber, and single-walled carbon nanotubes, and the weight ratio of the conductive carbon black, carbon fiber, and single-walled carbon nanotubes is 1: (0.25-0.3): (0.02-0.03); or, the conductive agent includes conductive carbon black, multi-walled carbon nanotubes, carbon fiber, and single-walled carbon nanotubes, and the weight ratio of the conductive carbon black, multi-walled carbon nanotubes, carbon fiber, and single-walled carbon nanotubes is 1: 0.5: (0.4-0.5): (0.01-0.02). With respect to the selection of the conductive agent, the inventors have selected the above two specific conductive agent systems through a large number of experiments. When the slurry is obtained and coated to form the positive active layer, a more stable three-dimensional conductive network can be constructed, and the positive slurry system can be more effectively adapted to improve its stability, and ultimately more significantly improve the various performances of the obtained lithium-ion battery.

[0027] In order to further improve the dispersibility and stability of the obtained positive electrode material slurry, it is further preferred that step S2-2 includes: adding the first carbon material to the first slurry, mixing at a revolution speed of 10rpm to 25rpm for 10min to 1h to obtain a mixed slurry; adding the second carbon material to the mixed slurry, mixing at a revolution speed of 10rpm to 25rpm and a rotation speed of 1800rpm to 1900rpm to obtain a second slurry. When the first carbon material (zero-dimensional carbon material, such as conductive carbon black) is added to the first slurry (slurry formed by dissolving the binder in the solvent), mixing at a revolution speed of 10rpm to 25rpm can more effectively promote the uniform dispersion of the first carbon material in the slurry, thereby forming conductive points on the surface of the positive electrode material particles, providing attachment points for the subsequent second carbon material (one-dimensional and / or two-dimensional carbon material). The second carbon material is added to the slurry formed by the preliminary mixing of the first carbon material and the binder, and two-stage mixing is performed, the revolution speed is maintained at 10rpm to 25rpm, and the rotation speed is increased to 1800rpm to 1900rpm. In this process, the special structural characteristics of one-dimensional carbon materials (such as carbon nanotubes) and / or two-dimensional carbon materials (such as graphene) are used to further optimize the conductive network in the slurry, and at the same time, the second carbon material is allowed to enter the slurry and achieve full mixing, and the excessive shearing of the slurry during this mixing process is reduced, and finally a positive electrode material slurry with higher stability and processability is obtained, which more effectively optimizes the stability of the lithium-ion battery obtained by subsequent coating.

[0028] When the second carbon material includes multi-walled carbon nanotubes, the preferred step S2-2 includes: forming a composite conductive slurry with a solid content of 9wt% to 10wt% with the multi-walled carbon nanotubes and the first carbon material, and then adding it to the first slurry, and mixing it in a stage with a revolution speed of 10rpm to 15rpm and a time of 10min to 15min to obtain a mixed slurry; adding the remaining second carbon material to the mixed slurry, and mixing it in a second stage with a revolution speed of 10rpm to 25rpm and a rotation speed of 1800rpm to 1900rpm to obtain a second slurry. Since multi-walled carbon nanotubes have a multi-layer graphene wall structure. It can better resist the mechanical stress and chemical corrosion that may occur during the battery charge and discharge cycle, while zero-dimensional carbon materials such as conductive carbon black can provide a large number of contact points. Pre-forming these two carbon materials into a composite conductive slurry with a specific solid content can promote the formation of a more stable mixed structure between multi-walled carbon nanotubes and conductive carbon black, thereby building a more complex and effective conductive network in the positive electrode material, improving the conductivity, and reducing the internal resistance of the battery. When the above conditions are adopted for one-stage mixing to obtain a one-stage mixed slurry, the multi-walled carbon nanotubes and the first carbon material can be initially dispersed under relatively low shear force, while the rheological properties of the slurry are more effectively maintained and its stability is improved.

[0029] In practical applications, the above-mentioned composite conductive paste, in addition to multi-walled carbon nanotubes and the first carbon material, also includes a small amount of dispersant to improve the uniformity and stability of the conductive paste, such as PVP, PEG and PAA, and the solid content of 9wt% to 10wt% specifically refers to the weight ratio of multi-walled carbon nanotubes and the first carbon material.

[0030] Furthermore, in order to obtain a more uniformly dispersed and more stable slurry system, the dispersant is preferably selected from at least two of a polycyclic dispersant, a phosphoric acid dispersant, an aliphatic hydrocarbon dispersant, a polyether dispersant, and an amide dispersant. Among them, the amide group of the amide dispersant can be adsorbed on the carbon-coated area on the surface of the positive electrode material, while the phosphate group of the phosphoric acid dispersant can be adsorbed on the non-carbon-coated area on the surface of the material, and the solvated chain segment of the polyether dispersant / polyester dispersant / aliphatic hydrocarbon dispersant can utilize the steric hindrance effect to isolate the nano-sized positive electrode material particles to prevent them from agglomerating and gelling.

[0031] On this basis, since the polycyclic dispersant can coat active particles through its unique molecular framework, reduce its surface energy and prevent agglomeration. Polyether dispersants can provide additional hydrogen bond donors, enhance mutual repulsion between particles, and improve dispersion stability. Amide dispersants have excellent intermolecular forces and help to form a stable dispersion system. When the three of them work together, the dispersion effect can be particularly significantly improved, so that each component is more evenly and stably dispersed in the slurry. Therefore, more preferably, the dispersant is selected from at least four of polycyclic dispersants, phosphoric acid dispersants, aliphatic hydrocarbon dispersants, polyether dispersants and amide dispersants, and the dispersant includes polycyclic dispersants, polyether dispersants and amide dispersants.

[0032] In several typical embodiments, the dispersant is a mixed dispersant formed by a polycyclic dispersant, a phosphoric acid dispersant, a polyether dispersant and an amide dispersant, and the weight ratio of the polycyclic dispersant, the phosphoric acid dispersant, the polyether dispersant and the amide dispersant is 1: (0.4-0.6): 1.5: 2. In addition to the polycyclic dispersants, polyether dispersants and amide dispersants mentioned above, a phosphoric acid dispersant is additionally introduced into this dispersant formula, which can form a chemical bond with the active material through the polar groups in the molecular structure, thereby enhancing the dispersion stability and conductivity of the particles. When the above dispersants are mixed in the above-mentioned specific proportions, a synergistic effect can be produced to significantly improve the dispersion efficiency of each component in the slurry, reduce agglomeration, and form a more uniform slurry with superior electrochemical performance after curing.

[0033] Alternatively, in several other typical embodiments, the dispersant is a mixed dispersant formed by a polycyclic dispersant, an aliphatic hydrocarbon dispersant, a polyether dispersant and an amide dispersant, and the weight ratio of the polycyclic dispersant, the aliphatic hydrocarbon dispersant, the polyether dispersant and the amide dispersant is 1: (0.4-0.6): 1.5: 2. In this dispersant formula, the additionally introduced aliphatic hydrocarbon dispersant can reduce the surface energy of the active material through its hydrophobicity and maintain a more uniform distribution of particles in the slurry. The combination of these four dispersants according to the above-mentioned weight relationship can synergize from different angles to achieve better dispersion effect and rheological control, so that the resulting positive electrode material slurry is more evenly dispersed, and at the same time more stable and not easy to stratify.

[0034] As for the specific types of the above dispersants, it is preferred that: the polycyclic dispersant is cyclodextrin; and / or the phosphoric acid dispersant is triethyl phosphate and / or triphenyl phosphate; and / or the aliphatic hydrocarbon dispersant is selected from one or more of n-hexadecane, isohexadecane and n-dodecane; and / or the polyether dispersant is selected from one or more of polyethylene glycol, polypropylene glycol and polytetrahydrofuran; and / or the amide dispersant is selected from one or more of polyacrylamide, N,N-dimethylformamide and N-vinylpyrrolidone. Among them, the cyclodextrin is selected from α-cyclodextrin (C 36 H 60 O 30 ), β-cyclodextrin (C 42 H 70 O 35 ) and γ-cyclodextrin (C 48 H 80 O 40 )

[0035] In several more typical embodiments, the dispersant is a mixed dispersant formed by cyclodextrin, triethyl phosphate, polyethylene glycol and polyacrylamide in a weight ratio of 1: (0.48-0.50): 1.5: 2; or, the dispersant is a mixed dispersant formed by cyclodextrin, hexadecane, polyethylene glycol and polyacrylamide in a weight ratio of 1: (0.48-0.50): 1.5: 2. After a large number of experiments, the inventors mixed cyclodextrin, triethyl phosphate (or hexadecane), polyethylene glycol and polyacrylamide as dispersants in a specific weight ratio, wherein cyclodextrin can encapsulate other small molecules through its cyclic structure to improve dispersion stability; triethyl phosphate and hexadecane are used as solvents to enhance the separation between particles; polyethylene glycol and polyacrylamide provide good rheological properties and viscosity control, and under synergistic effect, the particles in the slurry are more evenly dispersed and the stability is significantly improved. The above two preferred specific dispersants can more significantly optimize the rheological properties of the slurry, that is, improve the fluidity and shear behavior of the slurry. At the same time, the agglomeration and stratification of active materials in the slurry are more effectively reduced, thereby improving the consistency and yield rate of subsequently prepared batteries.

[0036] Further, in step S2, the first mixing, the second mixing, the third mixing, the fourth mixing, the fifth mixing and the sixth mixing are all carried out by a double planetary mixer, which rotates around the central axis and its own axis at the same time through the planetary cutter, generating a strong shear force and mixing effect. This mixing method can promote the uniform dispersion of the components in the positive electrode slurry, especially for the high solid content slurry system provided by the present invention, which can effectively reduce the agglomeration and stratification phenomenon, and improve the uniformity and stability of the slurry. And, the revolution speed of the first mixing, the fifth mixing and the sixth mixing is independently 20rpm~40rpm, preferably 20rpm~25rpm, and the rotation speed is independently 1000rpm~2000rpm, preferably 1900rpm~2000rpm. In the process of the first mixing, the fifth mixing and the sixth mixing, the above-mentioned preferred stirring speed combination can promote the mixing effect of each component to be more uniform, and can also more effectively reduce the material structure damage caused by excessive shearing. Preferably, the first mixing time is 2.5h to 3.0h; and / or the fifth mixing time is 3.5h to 4.0h; and / or the sixth mixing time is 0.5h to 1.0h. This helps to ensure sufficient and uniform mixing of the components without introducing additional shear force due to prolonged stirring and causing changes in the properties of the slurry.

[0037] Specifically, the solid content of the fifth slurry obtained after the fifth mixing is preferably 65wt% to 70wt%, more preferably 68wt% to 70wt%, so as to better balance the viscosity and fluidity of the slurry, thereby improving the uniformity and consistency of the obtained slurry during the coating process, and then obtaining a lithium-ion battery with higher cycle stability. And, for the mixed slurry obtained after the sixth mixing, the viscosity is preferably 4000mpa·s to 6000mpa·s, more preferably 5000±200mpa·s. Within the above preferred and more preferred viscosity range, the interaction force between the slurry components (including lithium manganese iron phosphate, conductive agent, binder and solvent) is more balanced, which helps to more effectively inhibit the stratification and precipitation of the obtained mixed slurry during subsequent treatment and the standing or transportation process after treatment. At the same time, the above viscosity range also helps to form a more stable suspension system, reduce the particle settling rate, thereby improving the long-term stability of the slurry and the cycle stability of the pole piece obtained by coating.

[0038] In several typical embodiments, step S2-3 includes: adding part A of the positive electrode material to the second slurry, and obtaining a third slurry through a third mixing with a revolution speed of 10rpm to 15rpm and a time of 10min to 15min; adding part B of the positive electrode material to the third slurry, and obtaining a fourth slurry through a fourth mixing with a revolution speed of 10rpm to 15rpm and a time of 0.5h to 1h. Because the positive electrode material lithium manganese iron phosphate has a certain tendency to agglomerate, the preparation method provided by the present invention divides it into two parts, A and B, and gradually adds it, which can reduce the single addition amount, avoid the local high concentration caused by adding a large amount of positive electrode material at one time, thereby reducing the agglomeration phenomenon and ensuring that the material is more evenly dispersed in the slurry. And further preferably, the conditions for adding and mixing twice are as above, so that the viscosity and solid content of the slurry in the preparation process can be more finely controlled. After adding part A of the positive electrode material, the rheological properties of the slurry can be preliminarily adjusted through the third mixing process to make its viscosity moderate. Then, part B of the positive electrode material is added, and the fourth mixing is continued to make the solid content of the slurry 68wt~70wt%. This solid content is conducive to the subsequent coating, and can also more effectively protect the structure of the positive electrode material and extend the storage stability of the obtained slurry.

[0039] The second aspect of the present invention provides a lithium iron manganese phosphate positive electrode material slurry, which is prepared by the above-mentioned method for preparing the lithium iron manganese phosphate positive electrode material slurry. The positive electrode material slurry prepared by the above-mentioned method has significantly improved electrochemical performance and stability, and can be more effectively adapted to the coating and preparation of battery pole pieces, thereby obtaining a high-performance lithium-ion battery.

[0040] It should be noted that due to the complexity of the slurry system and the limitations of existing test characterization methods, it is difficult to conduct a comprehensive quantitative characterization of the physicochemical characteristics of the above-mentioned lithium iron manganese phosphate positive electrode material slurry. However, after coating and hot pressing, the obtained positive electrode sheet and the corresponding lithium-ion battery have better cycle stability.

[0041] Furthermore, with respect to the stability of the obtained lithium iron manganese phosphate positive electrode material slurry, at 25±2°C, the viscosity of the lithium iron manganese phosphate positive electrode material slurry is recorded as V1; the viscosity of the lithium iron manganese phosphate positive electrode material slurry after standing for 24 hours is recorded as V2; (V2-V1) / V1×100%=78%-160%. In other words, the positive electrode material slurry obtained by the present invention has high stability, and after standing for a long time, the viscosity change rate is low, and no gel will appear, so that the coated lithium ion battery can more effectively maintain high electrical performance and cycle stability.

[0042] The third aspect of the present invention provides a lithium-ion battery, including a positive electrode sheet, and the positive electrode sheet includes a positive current collector and a positive active layer arranged on at least one side of the positive current collector, and the positive active layer is obtained by coating, drying and hot pressing the above-mentioned lithium iron manganese phosphate positive electrode material slurry. The lithium-ion battery prepared by coating the above-mentioned slurry provided by the present invention has small polarization, good conductivity, good cycle performance, and is particularly suitable for promotion and application in the field of new energy vehicles.

[0043] The present application is further described in detail below in conjunction with specific embodiments. These embodiments should not be construed as limiting the scope of protection claimed in the present application.

[0044] Unless otherwise defined, all professional terms used below have the same meanings as those generally understood by those skilled in the art. The professional terms used herein are only for the purpose of describing specific embodiments and are not intended to limit the scope of protection of the present invention.

[0045] Example 1

[0046] A method for preparing lithium manganese iron phosphate positive electrode material slurry:

[0047] (1) Ingredients:

[0048] Prepare relevant materials and set aside according to the ratio of 96%: 1.75%: 0.5%: 0.05%: 1.7%: 0.4% of lithium manganese iron phosphate positive electrode material, conductive carbon black, carbon fiber (VGCF), single-walled carbon nanotubes, polyvinylidene fluoride (i.e. binder, with a molecular weight greater than 800,000), and multi-polymer composite superdispersant.

[0049] The first carbon material is conductive carbon black, the second carbon material is carbon fiber and single-walled carbon nanotube, the first carbon material: the second carbon material = 3.2:1 (conductive carbon black: carbon fiber: single-walled carbon nanotube = 1:0.29:0.029); the single-walled carbon nanotube is added in the form of a slurry with a solid content of 0.4wt%. The multi-polymer composite super dispersant is a mixed dispersant formed by cyclodextrin (polycyclic): triethyl phosphate (phosphoric acid): polyethylene glycol (polyether): polyacrylamide (amide) = 1:0.5:1.5:2.

[0050] (2) Mixing:

[0051] (2-1) Add polyvinylidene fluoride and nitrogen-methylpyrrolidone (NMP) solvent in a weight ratio of 1:19 into a double planetary mixer (taking 200L as an example), and perform a first mixing to prepare a polyvinylidene fluoride glue solution with a solid content of 5wt%, i.e., a first slurry. The first mixing has an orbital speed of 25rpm, a rotation speed of 1900rpm, and a stirring time of 3h.

[0052] (2-2) Add conductive carbon black to the first slurry, stir at an orbital speed of 10 rpm for 10 min to obtain a first-stage mixed slurry; then add VGCF to the first-stage mixed slurry, stir at an orbital speed of 10 rpm for 10 min, and then stir at an orbital speed of 25 rpm and a rotation speed of 1900 rpm for 1.5 h to obtain a second-stage mixed slurry; finally, add single-walled carbon nanotube slurry to the second-stage mixed slurry, stir at an orbital speed of 25 rpm and a rotation speed of 1900 rpm for 1 h to obtain a second slurry.

[0053] (2-3) The lithium manganese iron phosphate positive electrode material is evenly divided into a positive electrode material part A and a positive electrode material part B; the positive electrode material part A is added to the second slurry, and stirred at an orbital speed of 10 rpm for 10 min (i.e., the third mixing) to obtain a third slurry; the remaining positive electrode material part B is added to the third slurry, and stirred at an orbital speed of 10 rpm for 0.5 h (i.e., the fourth mixing) to obtain a slurry with a solid content of 69 wt%, i.e., the fourth slurry.

[0054] (2-4) A dispersant was added to the fourth slurry, and a fifth mixing was performed at an orbital speed of 25 rpm and a rotational speed of 1900 rpm for 4 hours to obtain a slurry with a solid content of 68.5 wt%, i.e., the fifth slurry.

[0055] (2-5) Add the remaining nitrogen-methylpyrrolidone solvent to the fifth slurry, start the planetary mixer for stirring, and perform the sixth mixing at an orbital speed of 25 rpm and a rotation speed of 1900 rpm for 0.5 h. Adjust the slurry viscosity to 5000 mPa·s to obtain a mixed slurry.

[0056] (3) Post-treatment: The obtained mixed slurry was evacuated, and the stirring blade of the planetary mixer was reversed to perform vacuum defoaming at a revolution speed of 10 rpm. After removing small bubbles in the mixed slurry, the material was sieved through a 150-mesh sieve to obtain a lithium manganese iron phosphate positive electrode material slurry with a solid content of 64.5 wt%.

[0057] In the above preparation method, due to the complex actual production situation and environmental conditions, the solid content of the slurry obtained in each step fluctuates slightly within a narrow range, about ±1.0wt%, and the above solid content values ​​are all intermediate values ​​within the fluctuation range. The same below.

[0058] Example 2

[0059] A method for preparing lithium manganese iron phosphate positive electrode material slurry:

[0060] (1) Ingredients:

[0061] Prepare relevant materials according to the ratio of lithium manganese iron phosphate positive electrode material, composite conductive slurry (composite slurry of conductive carbon black and multi-walled carbon nanotubes, solid content is 9wt%, and conductive carbon black: multi-walled carbon nanotubes = 2:1, weight ratio), VGCF, single-walled carbon nanotubes, polyvinylidene fluoride (i.e. binder, whose molecular weight is greater than 800,000), and multi-copolymer composite super dispersant of 96.3%: 1.5%: 0.48%: 0.02%: 1.7%: 0.4% for use.

[0062] The first carbon material is conductive carbon black, the second carbon material is carbon fiber, multi-walled carbon nanotube and single-walled carbon nanotube, the first carbon material: the second carbon material = 1:1 (conductive carbon black: multi-walled carbon nanotube: carbon fiber: single-walled carbon nanotube = 1.0%: 0.5%: 0.48%: 0.02%); the single-walled carbon nanotube is added in the form of a slurry with a solid content of 0.4wt%. The multi-polymer composite super dispersant is a mixed dispersant formed by cyclodextrin (polycyclic): n-hexadecane (aliphatic hydrocarbon): polyethylene glycol (polyether): polyacrylamide (amide) = 1:0.5:1.5:2.

[0063] (2) Mixing:

[0064] (2-1) Add polyvinylidene fluoride and nitrogen-methylpyrrolidone (NMP) solvent in a weight ratio of 1:20 into a double planetary mixer (taking 100 L as an example), and perform a first mixing to prepare a polyvinylidene fluoride glue solution with a solid content of 5 wt%, i.e., a first slurry. The first mixing has an orbital speed of 20 rpm, a rotation speed of 2000 rpm, and a stirring time of 2.5 h.

[0065] (2-2) Add the composite conductive slurry to the first slurry, stir at an orbital speed of 25 rpm and a rotational speed of 1900 rpm for 1 hour to obtain a first-stage mixed slurry; then add VGCF to the first-stage mixed slurry, stir at an orbital speed of 10 rpm for 10 minutes, and then stir at an orbital speed of 25 rpm and a rotational speed of 1800 rpm for 1 hour to obtain a second-stage mixed slurry; finally, add the carbon nanotube slurry to the second-stage mixed slurry, stir at an orbital speed of 25 rpm and a rotational speed of 1900 rpm for 1 hour to obtain a second slurry.

[0066] (2-3) The lithium manganese iron phosphate positive electrode material is evenly divided into a positive electrode material part A and a positive electrode material part B; the positive electrode material part A is added to the second slurry, and stirred at an orbital speed of 15 rpm for 15 minutes (i.e., the third mixing) to obtain a third slurry; the remaining positive electrode material part B is added to the third slurry, and stirred at an orbital speed of 15 rpm for 1 hour (i.e., the fourth mixing) to obtain a slurry with a solid content of 69.5 wt%, i.e., the fourth slurry.

[0067] (2-4) A dispersant was added to the fourth slurry, and a fifth mixing was performed at an orbital speed of 40 rpm and a rotational speed of 2000 rpm for 3.5 hours to obtain a slurry with a solid content of 69 wt%, i.e., the fifth slurry.

[0068] (2-5) Add the remaining nitrogen-methylpyrrolidone solvent to the fifth slurry, start the planetary mixer for stirring, and perform the sixth mixing at an orbital speed of 10 rpm and a rotation speed of 1000 rpm for 1 hour. Adjust the slurry viscosity to 5000 mPa·s to obtain a mixed slurry.

[0069] (3) Post-treatment: The obtained mixed slurry was evacuated, and the stirring blade of the planetary mixer was reversed to perform vacuum defoaming at a revolution speed of 10 rpm. After removing small bubbles in the mixed slurry, the material was sieved through a 150-mesh sieve to obtain a lithium manganese iron phosphate positive electrode material slurry with a solid content of 64.5 wt%.

[0070] Example 3

[0071] A method for preparing lithium manganese iron phosphate positive electrode material slurry:

[0072] The difference between this embodiment and embodiment 1 is that the amount of each component is changed to:

[0073] Lithium manganese iron phosphate positive electrode material: conductive carbon black: carbon fiber: single-walled carbon nanotube: polyvinylidene fluoride: multi-copolymer composite superdispersant = 96%: 0.2%: 1.8%: 0.3%: 1.7%: 0.4%; at this time, the weight ratio of the first carbon material and the second carbon material is changed to 0.095:1.

[0074] Example 4

[0075] A method for preparing lithium manganese iron phosphate positive electrode material slurry:

[0076] The difference between this embodiment and embodiment 1 is that the amount of each component is changed to:

[0077] Lithium manganese iron phosphate positive electrode material: conductive carbon black: carbon fiber: single-walled carbon nanotube: polyvinylidene fluoride: multi-copolymer composite superdispersant = 96%: 1.9%: 0.3%: 0.1%: 1.7%: 0.4%; at this time, the weight ratio of the first carbon material and the second carbon material is changed to 4.75:1.

[0078] Example 5

[0079] A method for preparing lithium manganese iron phosphate positive electrode material slurry:

[0080] The difference between this embodiment and embodiment 1 is only in step (2-2), specifically:

[0081] (2-2) Add conductive carbon black, VGCF and single-walled carbon nanotube slurry into the first slurry, and stir at an orbital speed of 25 rpm and a rotational speed of 1900 rpm for 1 hour to obtain a second slurry.

[0082] Example 6

[0083] A method for preparing lithium manganese iron phosphate positive electrode material slurry:

[0084] The difference between this embodiment and embodiment 1 is only in step (2-2), specifically:

[0085] (2-2) Add conductive carbon black to the first slurry, stir at an orbital speed of 5 rpm for 10 minutes to obtain a first-stage mixed slurry; then add VGCF to the first-stage mixed slurry, stir at an orbital speed of 5 rpm for 10 minutes, and then stir at an orbital speed of 30 rpm and a rotation speed of 2500 rpm for 1.5 hours to obtain a second-stage mixed slurry; finally, add single-walled carbon nanotube slurry to the second-stage mixed slurry, stir at an orbital speed of 5 rpm and a rotation speed of 1500 rpm for 1 hour to obtain a second slurry.

[0086] Example 7

[0087] A method for preparing lithium manganese iron phosphate positive electrode material slurry:

[0088] The difference between this embodiment and embodiment 1 is that the multi-component copolymerized composite superdispersant is changed to:

[0089] A mixed dispersant formed by cyclodextrin (polycyclic type): triethyl phosphate (phosphoric acid type): polyethylene glycol (polyether type): polyacrylamide (amide type) = 1:1:1.5:2.

[0090] Example 8

[0091] A method for preparing lithium manganese iron phosphate positive electrode material slurry:

[0092] The difference between this embodiment and embodiment 2 is that the multi-component copolymerized composite superdispersant is changed to:

[0093] A mixed dispersant formed by cyclodextrin (polycyclic type): n-hexadecane (aliphatic hydrocarbon type): polyethylene glycol (polyether type): polyacrylamide (amide type) = 1:0.2:1.5:2.

[0094] Example 9

[0095] A method for preparing lithium manganese iron phosphate positive electrode material slurry:

[0096] The only difference between this embodiment and embodiment 1 is that the multi-polymer composite super dispersant used does not contain triethyl phosphate (phosphoric acid type) and polyethylene glycol (polyether type), but only a mixed dispersant formed by cyclodextrin (polycyclic type): polyacrylamide (amide type) = 1:2.

[0097] Example 10

[0098] A method for preparing lithium manganese iron phosphate positive electrode material slurry:

[0099] The difference between this embodiment and embodiment 1 is only in step (2-3), specifically:

[0100] (2-3) The lithium manganese iron phosphate positive electrode material is divided into a positive electrode material part A and a positive electrode material part B (and the positive electrode material part A: the positive electrode material part B = 2:1, weight ratio); the positive electrode material part A is added to the second slurry, and stirred at an orbital speed of 25 rpm for 5 minutes (i.e., the third mixing) to obtain a third slurry; the remaining positive electrode material part B is added to the third slurry, and stirred at an orbital speed of 5 rpm for 2 hours (i.e., the fourth mixing) to obtain a fourth slurry.

[0101] Comparative Example 1

[0102] A method for preparing lithium manganese iron phosphate positive electrode material slurry:

[0103] The only difference between this comparative example and Example 1 is that no dispersant is added.

[0104] Comparative Example 2

[0105] A method for preparing lithium manganese iron phosphate positive electrode material slurry:

[0106] The only difference between this comparative example and Example 2 is that no dispersant is added.

[0107] Comparative Example 3

[0108] A method for preparing lithium manganese iron phosphate positive electrode material slurry:

[0109] The only difference between this comparative example and Example 2 is that the weight percentage of the dispersant is changed from 0.4% to 1.0%.

[0110] Comparative Example 4

[0111] A method for preparing lithium manganese iron phosphate positive electrode material slurry:

[0112] The difference between this comparative example and Example 1 is only in step (2-3), specifically:

[0113] (2-3) All the lithium manganese iron phosphate positive electrode material was added to the second slurry, and stirred at a revolution speed of 10 rpm for 0.5 h to obtain a fourth slurry.

[0114] Comparative Example 5

[0115] A method for preparing lithium manganese iron phosphate positive electrode material slurry:

[0116] The difference between this comparative example and Example 1 is that in step (2), all components are mixed synchronously and stirred for 0.5 h at an orbital speed of 25 rpm and a rotational speed of 1900 rpm to obtain a mixed slurry.

[0117] Test Method

[0118] Changes in viscosity of the slurry after standing for 0h to 24h: The viscosity tester uses an NDJ-5S digital display rotational viscometer, and the test method is as follows: A) Put the slurry prepared in each embodiment and comparative example into a test beaker, and then test it in an environment with a temperature of 25±2℃ and a dew point ≤-35℃; B) Select rotor No. 4, adjust the speed to 30rpm (when the range is insufficient, reduce the speed to 10rpm), and the test time is 60s; C) Slowly insert rotor No. 4 into the center area of ​​the beaker and stop after reaching the predetermined depth; D) Start the test, read the digital viscosity value after 60s and record it. For the slurries obtained in the above embodiments and comparative examples, the viscosity is measured every 3h. The results are shown in Table 1. At the same time, the viscosity change curve of the slurries obtained in each embodiment and comparative example with the standing time is shown in Figure 1 .

[0119] Changes in the solid content of the upper and lower layers of slurry from 0h to 24h: The slurry obtained from each embodiment and comparative example is placed in a 100mL beaker, and the part where the slurry level is greater than the 50mL scale line is defined as the upper layer, and the part where the slurry level is less than the 50mL scale line is defined as the lower layer. The solid content of the upper and lower layers of slurry is measured every 6h. The specific test steps are as follows: A) Under an environment with a temperature of 25±2℃ and a dew point ≤-35℃, take a carrier (aluminum foil of 4cm*4cm size) and weigh it, and the weight is recorded as a; B) For each measurement, use a pipette to take 2mL of slurry from the upper and lower layers respectively, spread it evenly on the aluminum foil, weigh the weight of the aluminum foil with slurry, and record it as b; C) Bake the aluminum foil coated with slurry in an oven at 150℃ for 15min. After baking, weigh the weight and record it as c; D) Solid content = (ca) / (ba). The results are shown in Table 2. At the same time, the solid content of the upper and lower layers of the slurry obtained in each embodiment and comparative example varies with the standing time. Figure 2 .

[0120] The slurry fineness at 0h and 24h: The fineness test was carried out using a scraper fineness meter with a range of 50μm. The specific steps are as follows: A) The slurry prepared in each embodiment and comparative example was placed in a test beaker, and then tested in an environment with a temperature of 25±2℃ and a dew point ≤-35℃; B) The scraper fineness meter was cleaned with alcohol and placed horizontally on the table after cleaning; C) 2mL of slurry was taken with a test tube and placed in the deepest part of the fineness meter. The slurry was filled with The grooves are filled; D) Hold the scraper with both hands and place it horizontally on the upper end of the polished plate (at the edge of the sample) so that the scraper is in vertical contact with the surface of the polished plate. Within 3 seconds, pull the scraper from the deep part of the groove to the shallow part, so that the slurry fills the grooves and no slurry is left on the plate; E) After the scraper is pulled, immediately (no more than 5 seconds) make the line of sight at an angle of 15 to 30 degrees with the groove plane, observe the light at the place where the particles are evenly exposed in the groove, and record the readings (accurate to the minimum graduation value). If individual particles are exposed on other graduation lines, the readings shall not exceed three particles within the range of the adjacent graduation lines. The smaller the fineness, the better the dispersion effect. At the same time, in order to avoid errors as much as possible, for each embodiment and comparative example, parallel sampling and testing are carried out three times and the average value is taken.

[0121] Preparation of battery samples and cycle performance test: conventional graphite was used as the negative electrode material to prepare the negative electrode slurry (wherein graphite negative electrode material: conductive agent: binder: dispersant = 96.5%: 0.5%: 1.8%: 1.2%), and the positive electrode material slurry obtained in each embodiment and comparative example was used as the positive electrode slurry; copper foil was used as the negative electrode current collector, carbon-coated aluminum foil was used as the positive electrode current collector, and the positive electrode 387g / m 2 、Negative electrode 174g / m 2 The two slurries were coated on one side of the corresponding current collector with a coating surface density of , and the positive and negative electrodes were formed after drying. After obtaining the positive and negative electrodes, a battery sample with a capacity of 188.5Ah was assembled using lithium hexafluorophosphate solution as the electrolyte. At 25°C, within the voltage range of 2.5V to 4.25V, each battery sample was subjected to a 1C charge and discharge cycle. After 500 cycles, the discharge capacity retention rate of each battery sample was obtained.

[0122] The test results of slurry fineness and battery sample cycle performance are shown in Table 3. Also, the viscosity of lithium manganese iron phosphate positive electrode material slurry at 0h is recorded as V1; the viscosity of lithium manganese iron phosphate positive electrode material slurry after standing for 24h is recorded as V2, and the viscosity change rate after standing for 24h is defined as (V2-V1) / V1×100%. The performance results are also shown in Table 3.

[0123] Table 1

[0124]

[0125]

[0126] For the positive electrode material slurry, whether it is stable can be determined by the degree of change in viscosity. If the viscosity increases too fast, it means that the slurry properties are unstable, gel has appeared, and subsequent processing is very difficult, which in turn affects the various performances of the prepared battery. As can be seen from Table 1, the viscosity of Examples 1 and 2 is very stable, and through actual observation, there is no gel phenomenon. Among them, Comparative Example 3 adds excessive dispersant, and the processability is not much different from that of Examples 1 and 2. However, excessive addition of dispersant will cause the cycle performance of the corresponding battery to deteriorate, so the amount added in Examples 1 and 2 is more conducive to the stability of the slurry properties.

[0127] Table 2

[0128]

[0129]

[0130] As shown in Table 2 and Figure 2 As shown, the solid content of the upper and lower layers of the slurries prepared in Examples 1 and 2 do not change much over time, indicating that the slurries have very good dispersibility and are not easy to settle. The solid content of the upper and lower layers of the slurries prepared in other embodiments, especially the comparative examples, gradually increases with the standing time. Among them, the difference in solid content between the upper and lower layers of the slurries prepared in Comparative Examples 4 and 5 is the largest. It can be found from Comparative Example 3 that the addition of excessive dispersant does not further improve the stability of the slurry, but makes it further worse, indicating that the slurry prepared according to the homogenization method and the amount of dispersant added of the present invention has better stability.

[0131] Table 3

[0132]

[0133] In Table 3, it can be seen that the slurry prepared by using Examples 1 and 2 has a smaller fineness, the viscosity change rate of the slurry after standing for 24 hours is also the smallest, and the capacity retention rate after 500 cycles is the highest. By comparing each embodiment with Comparative Examples 1 to 4, it can be found that the slurry prepared by each embodiment has a smaller fineness, more stable viscosity, and a higher capacity retention rate, which can fully illustrate that the slurry prepared by the homogenization method of the present invention has better dispersion performance, and the battery prepared by using the slurry of the present invention has better cycle performance.

[0134] With respect to each embodiment, specifically:

[0135] By comparing Examples 1 and 2 with Examples 3 and 4, it can be seen that the weight ratio of the first carbon material to the second carbon material is preferably (0.5-1.0): (0.5-2.0), which can construct a more continuous and multi-dimensional conductive network, thereby improving the fluidity of the slurry, inhibiting its gelation and agglomeration, and more effectively prolonging the stability of the slurry.

[0136] By comparing Examples 1 and 2 with Examples 5 and 6, it can be seen that by further optimizing the order of adding the conductive agent and the stirring conditions, the special structural properties of one-dimensional carbon materials (such as carbon nanotubes) and / or two-dimensional carbon materials (such as graphene) can be more effectively utilized, and ultimately a positive electrode material slurry with higher stability and processability is obtained, which more effectively optimizes the stability of the lithium-ion battery obtained by subsequent coating.

[0137] By comparing Examples 1 and 2 with Examples 7 to 9, it can be seen that the two dispersants preferred by the present invention, namely, a mixed dispersant formed by cyclodextrin (polycyclic): triethyl phosphate (phosphoric acid): polyethylene glycol (polyether): polyacrylamide (amide) = 1: (0.4-0.6): 1.5: 2, and a mixed dispersant formed by cyclodextrin (polycyclic): n-hexadecane (aliphatic hydrocarbon): polyethylene glycol (polyether): polyacrylamide (amide) = 1: (0.4-0.6): 1.5: 2, can particularly effectively exert the structural and performance advantages of different types of dispersants, and finally obtain a positive electrode slurry with particularly stable stability. In particular, it can be seen from Example 9 that for the slurry system and preparation method provided by the present invention, when the polycyclic dispersant, polyether dispersant and amide dispersant in the slurry act together, the dispersion effect can be particularly significantly improved, so that each component can be dispersed in the slurry for a long time, more evenly and more stably.

[0138] By comparing Example 1 with Example 10, it can be seen that when the conditions for adding and mixing the positive electrode material twice are further optimized, the viscosity and solid content of the slurry during the preparation process can be more finely controlled, and ultimately a battery sample with better stability can be obtained.

[0139] From the above description, it can be seen that the above embodiments of the present invention achieve the preparation of positive electrode material slurry with high stability. When the obtained slurry is coated and prepared into a lithium ion battery, the polarization of the corresponding lithium ion battery can be further optimized and its cycle performance can be improved.

[0140] It should be noted that the terms "first", "second", etc. in the specification and claims of the present application are used to distinguish similar objects, and are not necessarily used to describe a specific order or sequence. It should be understood that the terms used in this way can be interchangeable where appropriate, so that the embodiments of the present application described herein can be implemented in an order other than those described herein, for example.

[0141] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. For those skilled in the art, the present invention may have various modifications and variations. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention shall be included in the protection scope of the present invention.

Claims

1. A method for preparing lithium manganese iron phosphate positive electrode material slurry, characterized in that: include: Step S1, ingredients: in parts by weight, the lithium manganese iron phosphate positive electrode material slurry includes 94-98 parts of lithium manganese iron phosphate positive electrode material, 1-3 parts of conductive agent, 1-2.5 parts of binder, 0.1-0.5 parts of dispersant, and the balance is solvent; Step S2, mixing: Step S2-1, adding the binder to a portion of the solvent, and performing a first mixing to obtain a first slurry; Step S2-2, adding the conductive agent into the first slurry, and performing a second mixing to obtain a second slurry; Step S2-3, dividing the lithium manganese iron phosphate positive electrode material into a positive electrode material part A and a positive electrode material part B; adding the positive electrode material part A to the second slurry, and performing a third mixing to obtain a third slurry; adding the positive electrode material part B to the third slurry, and performing a fourth mixing to obtain a fourth slurry; Step S2-4, adding the dispersant to the fourth slurry, and performing a fifth mixing to obtain a fifth slurry; Step S2-5, adding another portion of the solvent to the fifth slurry, and performing a sixth mixing to obtain a mixed slurry; The weight ratio of the binder to a portion of the solvent is 1:(19-20); The weight ratio of the positive electrode material part A to the positive electrode material part B is 1:(1.0-1.2); step S3, post-treatment: the mixed slurry is subjected to vacuum defoaming treatment and filtration treatment in turn to obtain the lithium manganese iron phosphate positive electrode material slurry; the solid content of the lithium manganese iron phosphate positive electrode material slurry is 60%-70%.

2. The method for preparing lithium manganese iron phosphate positive electrode material slurry according to claim 1, characterized in that: The conductive agent includes a first carbon material and a second carbon material; the first carbon material is a zero-dimensional carbon material, and the second carbon material is a one-dimensional carbon material and / or a two-dimensional carbon material; Preferably, the zero-dimensional carbon material is conductive carbon black and / or conductive graphite; the one-dimensional carbon material is selected from one or more of carbon fiber, single-walled carbon nanotube and multi-walled carbon nanotube; the two-dimensional carbon material is graphene; More preferably, the solvent is nitrogen-methylpyrrolidone.

3. The method for preparing lithium manganese iron phosphate positive electrode material slurry according to claim 1 or 2, characterized in that: The weight ratio of the first carbon material to the second carbon material is (0.01-4.5):1; Preferably, the conductive agent includes the zero-dimensional carbon material and the one-dimensional carbon material, and the weight ratio of the zero-dimensional carbon material to the one-dimensional carbon material is (1-3.5):

1.

4. The method for preparing lithium manganese iron phosphate positive electrode material slurry according to claim 2 or 3, characterized in that: The step S2-2 comprises: The first carbon material is added to the first slurry, and mixed at a revolution speed of 10 rpm to 25 rpm for 10 min to 1 h to obtain a mixed slurry; The second carbon material is added to the first mixed slurry, and the second slurry is obtained by mixing in two stages with an orbital speed of 10 rpm to 25 rpm and a rotation speed of 1800 rpm to 1900 rpm.

5. The method for preparing lithium manganese iron phosphate positive electrode material slurry according to any one of claims 1 to 4, characterized in that: The dispersant is selected from at least two of a polycyclic dispersant, a phosphoric acid dispersant, an aliphatic hydrocarbon dispersant, a polyether dispersant and an amide dispersant; Preferably, the polycyclic dispersant is cyclodextrin; and / or, the phosphoric acid dispersant is triethyl phosphate and / or triphenyl phosphate; and / or, the aliphatic hydrocarbon dispersant is selected from one or more of n-hexadecane, isohexadecane and n-dodecane; and / or, the polyether dispersant is selected from one or more of polyethylene glycol, polypropylene glycol and polytetrahydrofuran; and / or, the amide dispersant is selected from one or more of polyacrylamide, N,N-dimethylformamide and N-vinylpyrrolidone; More preferably, the dispersant is selected from at least four of the polycyclic dispersants, the phosphoric acid dispersants, the aliphatic hydrocarbon dispersants, the polyether dispersants and the amide dispersants, and the dispersants include the polycyclic dispersants, the polyether dispersants and the amide dispersants.

6. The method for preparing lithium manganese iron phosphate positive electrode material slurry according to any one of claims 1 to 5, characterized in that: In step S2, the first mixing, the second mixing, the third mixing, the fourth mixing, the fifth mixing and the sixth mixing are all performed using a double planetary mixer, and: The revolution speed of the first mixing, the fifth mixing and the sixth mixing is independently 20 rpm to 40 rpm, preferably 20 rpm to 25 rpm, and the rotation speed is independently 1000 rpm to 2000 rpm, preferably 1900 rpm to 2000 rpm; Preferably, the first mixing time is 2.5h to 3.0h; And / or, the fifth mixing time is 3.5h to 4.0h; And / or, the sixth mixing time is 0.5h to 1.0h.

7. The method for preparing lithium manganese iron phosphate positive electrode material slurry according to any one of claims 1 to 6, characterized in that: The step S2-3 comprises: The positive electrode material A is partially added to the second slurry, and the third slurry is obtained through the third mixing at an orbital speed of 10rpm to 15rpm and a time of 10min to 15min; the positive electrode material B is partially added to the third slurry, and the fourth slurry is obtained through the fourth mixing at an orbital speed of 10rpm to 15rpm and a time of 0.5h to 1h.

8. A lithium manganese iron phosphate positive electrode material slurry, characterized in that: The lithium iron manganese phosphate positive electrode material slurry is prepared by the preparation method of the lithium iron manganese phosphate positive electrode material slurry according to any one of claims 1 to 7.

9. The lithium iron manganese phosphate positive electrode material slurry according to claim 8, characterized in that: At 25±2° C., the viscosity of the lithium manganese iron phosphate positive electrode material slurry is recorded as V1; the viscosity of the lithium manganese iron phosphate positive electrode material slurry after standing for 24 hours is recorded as V2; (V2-V1) / V1×100%=78%-160%.

10. A lithium-ion battery, comprising a positive electrode sheet, wherein the positive electrode sheet comprises a positive current collector and a positive active layer disposed on at least one side surface of the positive current collector, characterized in that: The positive electrode active layer is obtained by coating, drying and hot pressing the lithium manganese iron phosphate positive electrode material slurry as described in claim 8 or 9.

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