Positive electrode slurry, and homogenizing method therefor and use thereof

Abstract

A positive electrode slurry, and a homogenizing method therefor and the use thereof. The homogenizing method comprises: mixing the first part of a conductive agent, a binder and a polyphosphate auxiliary agent with the first part of a solvent, and subjecting the resulting mixture to first-step stirring; mixing same with the first part of a lithium iron phosphate positive electrode material and the second part of the conductive agent, and subjecting the resulting mixture to second-step stirring; and mixing same with the second part of the lithium iron phosphate positive electrode material until uniform, then adding the second part of the solvent thereto, and subjecting the resulting mixture to third-step stirring, so as to obtain a positive electrode slurry.

Description

Positive electrode slurry and homogenization method and application thereof

[0001] This application claims priority to the Chinese patent application filed with the China Patent Office on September 4, 2023, with application number 202311131219.0. The entire contents of the above application are incorporated by reference into this application. Technical Field

[0002] The present application belongs to the technical field of lithium-ion batteries and relates to a positive electrode slurry and a homogenization method and application thereof. Background Art

[0003] In related technologies, the materials of conventional lithium battery slurry usually include active materials, adhesives, conductive agents and solvents. Since the active materials account for a large proportion of the slurry, the output solid content, viscosity change, sedimentation and other properties are greatly affected by the active materials.

[0004] The preparation process for lithium-ion battery electrodes includes slurrying, coating, rolling, and cutting. The uniformity of the dispersion of the components in the battery slurry directly affects the processing performance of the electrode sheets, with the slurrying process accounting for over 30% of the battery performance impact. To improve slurrying efficiency, the active material is often modified. To adapt to different active materials, a matching slurry production process must be developed, which seriously affects production efficiency.

[0005] The surface of lithium iron phosphate is usually covered with a carbon layer. The surface of the covered carbon layer contains rich oxygen-containing functional groups. These oxygen-containing functional groups easily form hydrogen bonds with materials such as PVDF, thereby increasing the viscosity of the slurry. In order to ensure the subsequent processing performance of the slurry, the solid content of the output must be reduced, thereby increasing production costs.

[0006] The homogenization method used in related technologies is usually complicated and requires high operating power of the equipment. The solid content of the slurry produced is low and the dispersibility of the slurry is poor. Technical issues

[0007] The present application provides a positive electrode slurry and a homogenization method and application thereof to solve the above-mentioned technical problems. Technical Solutions

[0008] In a first aspect, an embodiment of the present application provides a method for homogenizing a positive electrode slurry, the method comprising the following steps:

[0009] The first part of the conductive agent, the binder and the polyphosphate additive are mixed with the first part of the solvent, and stirred in one step to obtain a first mixed material;

[0010] The first portion of the lithium iron phosphate positive electrode material and the second portion of the conductive agent are mixed with the first mixed material, and the second mixed material is obtained by stirring in two steps;

[0011] After the second portion of lithium iron phosphate positive electrode material and the second mixed material are evenly mixed, the second portion of solvent is added, and the positive electrode slurry is obtained through three steps of stirring.

[0012] In a second aspect, an embodiment of the present application provides a positive electrode slurry, which is prepared by the homogenization method as described above, and the positive electrode slurry contains a solid substance and a solvent.

[0013] In a third aspect, an embodiment of the present application provides a positive electrode sheet, which is obtained by coating the positive electrode slurry as described above on the surface of the positive electrode collector, drying it, and rolling it.

[0014] In a fourth aspect, an embodiment of the present application provides a lithium-ion battery, wherein the lithium-ion battery comprises the positive electrode sheet as described above. Beneficial effects

[0015] Beneficial effects of this application:

[0016] (1) Under the same stirring conditions, the homogenization method described in the present application increases the solid content of the slurry output, thereby improving production efficiency and reducing the amount of solvent used, greatly reducing production costs. The solid content of the obtained slurry is about 7% to 10% higher than that of conventional slurry, while the viscosity is the same as that of conventional slurry, which does not affect the subsequent processing performance of the slurry.

[0017] (2) The present application adds a polyphosphate ester auxiliary agent during the homogenization process. The polyphosphate ester auxiliary agent has the same phosphate group as that in lithium iron phosphate, has a stronger ability to bind to the particles, and has a better dispersing ability. It can effectively disperse the positive electrode particles and improve the uniformity and stability of the slurry.

[0018] (3) The positive electrode slurry prepared by the homogenization method described in this application has a viscosity of 15,000 to 18,000 MPa.s and a solid content of more than 69%. Increasing the dispersant can increase the solid content. Increasing the amount of dispersant added from 0.1% to 0.2% increases the solid content by 2 percentage points, and the viscosity does not change much. However, the addition of the dispersant has a significant effect on increasing the solid content. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] FIG1 is a schematic flow diagram of a method for homogenizing a positive electrode slurry according to an embodiment of the present application;

[0020] FIG2 is a schematic structural diagram of a lithium-ion battery provided in one embodiment of the present application;

[0021] FIG3 is a schematic structural diagram of the core package in FIG2 ;

[0022] FIG4 is a schematic structural diagram of the positive electrode sheet in FIG3 ;

[0023] Explanation of the accompanying reference numerals: 100 - positive electrode sheet; 10 - positive electrode current collector; 20 - coating; 200 - lithium-ion battery; 1 - outer shell; 11 - shell; 12 - shell cover; 2 - core pack; 3 - negative electrode sheet; 4 - diaphragm; 51 - positive electrode ear; 52 - positive electrode column; 53 - first connecting piece; 61 - negative electrode ear; 62 - negative electrode column; 63 - second connecting piece. Modes for Carrying Out the Invention

[0024] The purpose of the present application is to provide a positive electrode slurry and its homogenization method and application. The homogenization method described in the present application has a simple process, low equipment requirements, and the slurry has good dispersibility and high solid content, which can greatly improve production efficiency, save solvent usage, and reduce production costs.

[0025] To achieve the purpose of this invention, this application adopts the following technical solutions:

[0026] In a first aspect, an embodiment of the present application provides a method for homogenizing a positive electrode slurry. Referring to FIG1 , the homogenization method comprises the following steps:

[0027] S10, mixing the first portion of the conductive agent, the binder, and the polyphosphate additive with the first portion of the solvent, and stirring in one step to obtain a first mixed material;

[0028] S20, mixing the first portion of lithium iron phosphate positive electrode material and the second portion of conductive agent with the first mixed material, and stirring in two steps to obtain a second mixed material;

[0029] S30, after uniformly mixing the second portion of lithium iron phosphate positive electrode material and the second mixed material, adding the second portion of solvent, and stirring in three steps to obtain the positive electrode slurry.

[0030] It can be understood that the one-step stirring, two-step stirring and three-step stirring described in this application are the first stirring, the second stirring and the third stirring respectively.

[0031] It can be understood that the present application divides the conductive agent, lithium iron phosphate positive electrode material and solvent into two parts for feeding respectively. Specifically, the first part of the conductive agent and the second part of the conductive agent constitute the conductive agent, the first part of the lithium iron phosphate positive electrode material and the second part of the lithium iron phosphate positive electrode material constitute the lithium iron phosphate positive electrode material, and the first part of the solvent and the second part of the solvent constitute the solvent.

[0032] In the present application, a polyphosphate ester auxiliary agent is added during the homogenization process of the positive electrode slurry, and the homogenization is carried out in a step-by-step manner. The polyphosphate ester auxiliary agent contains an anchoring group and a solvation chain. The anchoring group of the polyphosphate ester auxiliary agent is a group containing phosphoric acid, which preferentially combines with the carbon layer on the surface of the lithium iron phosphate, thereby playing a very good anchoring role.

[0033] In step S10 of the present application, a conductive agent, a binder, a polyphosphate additive and a part of the solvent are pre-mixed to form a conductive adhesive (a first mixed material), so that the polyphosphate additive is evenly dispersed. During the stirring process of step S20, the phosphate group in the polyphosphate additive takes precedence over the binder as an anchoring group, and is firmly combined with the carbon layer on the surface of the lithium iron phosphate in the slurry, lithium iron phosphate and other substances to form a coating layer on the surface of the particles, while the remaining solvated chains remain in the solvent. Due to the mutual repulsion between the same solvated chains, the stability between the solid phase particles is improved.

[0034] In some embodiments, the conductive agent in step S10 includes any one of conductive carbon black, carbon nanotubes, or graphene, or a combination of at least two of them.

[0035] In some embodiments, the mass of the first portion of the conductive agent accounts for 20% to 60% of the total mass of the conductive agent, for example, 20%, 30%, 40%, 50% or 60%.

[0036] In some embodiments, the binder includes polyvinylidene fluoride.

[0037] In some embodiments, the solvent comprises N-methylpyrrolidone.

[0038] In some embodiments, the mass of the first portion of solvent accounts for 80-90% of the total mass of the solvent, for example, 80%, 82%, 85%, 88% or 90%.

[0039] In some embodiments, the stirring speed in step S10 is 1150-1250 rpm, for example, 1150 rpm, 1180 rpm, 1200 rpm, 1220 rpm or 1250 rpm.

[0040] In some embodiments, the stirring time for one step is 15 to 25 minutes, for example, 15 minutes, 18 minutes, 20 minutes, 22 minutes or 25 minutes.

[0041] In some embodiments, in step S20 , the mass of the first portion of lithium iron phosphate positive electrode material accounts for 40% to 60% of the total mass of the lithium iron phosphate positive electrode material, for example, 40%, 45%, 50%, 55% or 60%.

[0042] In some embodiments, the mass of the second portion of the conductive agent accounts for 40% to 80% of the total mass of the conductive agent, for example, 40%, 50%, 60%, 70% or 80%.

[0043] In some embodiments, the speed of the second stirring step in step S20 is 750-850 rpm, for example, 750 rpm, 780 rpm, 800 rpm, 820 rpm or 850 rpm.

[0044] In some embodiments, the second-step stirring time is 8 to 12 minutes, for example, 8 minutes, 9 minutes, 10 minutes, 11 minutes or 12 minutes.

[0045] In some embodiments, in step S30 , the mass of the second portion of lithium iron phosphate positive electrode material accounts for 40% to 60% of the total mass of the lithium iron phosphate positive electrode material, for example, 40%, 45%, 50%, 55% or 60%.

[0046] In some embodiments, the mass of the second portion of solvent accounts for 10-20% of the total mass of the solvent, for example, 10%, 12%, 15%, 18% or 20%.

[0047] In some embodiments, the speed of the three stirring steps in step S30 is 500-600 rpm, for example, 500 rpm, 520 rpm, 550 rpm, 580 rpm or 600 rpm.

[0048] In some embodiments, the vacuum degree of the three-step stirring is -0.09 to -0.08 MPa, for example: -0.09 MPa, -0.088 MPa, -0.085 MPa, -0.082 MPa or -0.08 MPa, etc.

[0049] In some embodiments, the three-step stirring time is 3 to 6 hours, for example, 3 hours, 3.5 hours, 4 hours, 5 hours or 6 hours.

[0050] In a second aspect, the present application provides a positive electrode slurry, which is prepared by the homogenization method as described in the first aspect, and the positive electrode slurry contains a solid substance and a solvent.

[0051] In some embodiments, the solid material includes a lithium iron phosphate positive electrode material, a binder, a conductive agent, and a polyphosphate additive.

[0052] In some embodiments, based on the mass of the solid substance being 100%, the mass fraction of the lithium iron phosphate positive electrode material is 95-99%, for example, 95%, 96%, 97%, 98% or 99%.

[0053] In some embodiments, based on the mass of the solid material being 100%, the mass fraction of the conductive agent is 0.3-0.5%, for example, 0.3%, 0.35%, 0.4%, 0.45% or 0.5%.

[0054] In some embodiments, based on the mass of the solid material being 100%, the mass fraction of the binder is 1-3%, for example, 1%, 1.5%, 2%, 2.5% or 3%.

[0055] In some embodiments, based on the mass of the solid matter being 100%, the mass fraction of the polyphosphate additive is 0.05-0.2%, for example, 0.05%, 0.08%, 0.1%, 0.15% or 0.2%.

[0056] In some embodiments, based on the mass of the positive electrode slurry being 100%, the mass fraction of the solvent is 30-50%, for example, 30%, 35%, 40%, 45% or 50%.

[0057] In some embodiments, the viscosity of the positive electrode slurry is 8000-30000 MPa.s, for example, 8000 MPa.s, 10000 MPa.s, 20000 MPa.s, 25000 MPa.s or 30000 MPa.s.

[0058] In some embodiments, the solid content of the positive electrode slurry is 65% to 70%, for example, 65%, 66%, 67%, 68% or 70%.

[0059] In a third aspect, the present application provides a positive electrode sheet 100 , which is obtained by coating the positive electrode slurry described in the second aspect on the surface of a positive electrode current collector 10 , followed by drying and roller pressing.

[0060] As shown in FIG4 , the positive electrode sheet 100 includes a positive electrode current collector 10 and a coating 20 disposed on one side of the positive electrode current collector 10. The positive electrode slurry described in the second aspect is applied to the surface of the positive electrode current collector 10, dried, and rolled to produce the positive electrode current collector 10 having the coating 20 disposed on the surface. Specifically, the material of the positive electrode current collector 10 can be any common conductive metal material in the art, including but not limited to aluminum foil, platinum foil, or palladium foil, and this application does not impose any limitation on this.

[0061] In a fourth aspect, the present application provides a lithium-ion battery 200, comprising the positive electrode sheet 100 as described in the third aspect. Specifically, referring to Figures 2 and 3, in some embodiments, the lithium-ion battery 200 comprises an outer shell 1 and a core pack 2, wherein the outer shell 1 is composed of a shell 11 and a shell cover 12, and a receiving space is formed within the shell 11, and the core pack 2 is installed within the receiving space. The core pack 2 comprises the positive electrode sheet 100, the separator 4, and the negative electrode sheet 3 stacked in sequence, wherein, in the positive electrode sheet 100, the positive current collector 10 is arranged close to the separator 4, and the coating 20 is arranged on the side of the positive current collector 10 facing away from the separator 4; in addition, in some embodiments, the core pack 2 further comprises a positive electrode tab 51 connected to the positive electrode sheet 100 and a negative electrode tab 61 connected to the negative electrode sheet 3. The core pack 2 is filled with an electrolyte. In addition, the lithium-ion battery 200 also includes a positive electrode column 52 and a negative electrode column 62 provided on the outer shell 1, wherein the positive electrode column 52 is provided corresponding to the positive electrode ear 51, and passes through the outer shell 1 and is connected to the positive electrode ear 51 through the first connecting piece 53, and the negative electrode column 62 is provided corresponding to the negative electrode ear 61, and passes through the outer shell 1 and is connected to the negative electrode ear 61 through the second connecting piece 63.

[0062] Unless otherwise specified, the ratios described in the examples and comparative examples of the present application are by mass ratios.

[0063] Example 1

[0064] This embodiment provides a method for homogenizing a positive electrode slurry, the method comprising the following steps:

[0065] (1) 40% conductive carbon black, polyvinylidene fluoride (PVDF) and polyphosphate additive were mixed with 85% NMP, and stirred at 1200 rpm for 20 minutes to obtain a first mixed material;

[0066] (2) mixing 50% of the lithium iron phosphate positive electrode material and the remaining 60% of the conductive carbon black with the first mixed material, and stirring at 800 rpm for 10 minutes to obtain a second mixed material;

[0067] (3) After the remaining 50% of the lithium iron phosphate positive electrode material and the second mixed material are evenly mixed, the remaining 15% of NMP is added, and the mixture is stirred at a speed of 550 rpm at -0.085 MPa for 4 hours to obtain the positive electrode slurry, which includes solid matter and solvent, the mass fraction of the solvent is 30%, and the mass fraction of the solid matter is 96.5% of the lithium iron phosphate positive electrode material, the mass fraction of the conductive carbon black is 0.4%, the mass fraction of PVDF is 3%, and the mass fraction of the polyphosphate additive is 0.1%.

[0068] Example 2

[0069] This embodiment provides a method for homogenizing a positive electrode slurry, the method comprising the following steps:

[0070] (1) 20% conductive carbon black, polyvinylidene fluoride (PVDF) and polyphosphate additive were mixed with 80% NMP, and stirred at 1150 rpm for 25 minutes to obtain a first mixed material; (2) 40% lithium iron phosphate positive electrode material and the remaining 80% conductive carbon black were mixed with the first mixed material, and stirred at 750 rpm for 12 minutes to obtain a second mixed material;

[0071] (3) After the remaining 60% of the lithium iron phosphate positive electrode material and the second mixed material are evenly mixed, the remaining 20% ​​of NMP is added, and the mixture is stirred at a speed of 500 rpm at -0.08 MPa for 6 hours to obtain the positive electrode slurry, which includes solid matter and solvent, the mass fraction of the solvent is 31%, and the mass fraction of the solid matter is 98% of the lithium iron phosphate positive electrode material, the mass fraction of the conductive carbon black is 0.4%, the mass fraction of PVDF is 1.5%, and the mass fraction of the polyphosphate additive is 0.1%.

[0072] Example 3

[0073] This embodiment provides a method for homogenizing a positive electrode slurry, the method comprising the following steps:

[0074] (1) 60% conductive carbon black, polyvinylidene fluoride (PVDF) and polyphosphate additive were mixed with 90% NMP, and stirred at 1250 rpm for 15 minutes to obtain a first mixed material;

[0075] (2) mixing 60% of the lithium iron phosphate positive electrode material and the remaining 40% of the conductive carbon black with the first mixed material, and stirring at 750 rpm for 12 minutes to obtain a second mixed material;

[0076] (3) After the remaining 40% of the lithium iron phosphate positive electrode material and the second mixed material are evenly mixed, the remaining 10% of NMP is added, and the mixture is stirred at a speed of 600 rpm at -0.09 MPa for 3 hours to obtain the positive electrode slurry, which includes solid matter and solvent, the mass fraction of the solvent is 28%, and the mass fraction of the solid matter is 97.5% of the lithium iron phosphate positive electrode material, the mass fraction of the conductive carbon black is 0.3%, the mass fraction of PVDF is 2%, and the mass fraction of the polyphosphate additive is 0.2%.

[0077] Example 4

[0078] The only difference between this embodiment and embodiment 1 is that, in the solid matter of the slurry, the mass fraction of the polyphosphate additive is 0.01%, and the mass fraction of the lithium iron phosphate positive electrode material is 96.59%. Other conditions and parameters are exactly the same as those in embodiment 1.

[0079] Example 5

[0080] The only difference between this embodiment and embodiment 1 is that, in the solid matter of the slurry, the mass fraction of the polyphosphate additive is 0.3%, and the mass fraction of the lithium iron phosphate positive electrode material is 96.3%. Other conditions and parameters are exactly the same as those in embodiment 1.

[0081] Comparative Example 1

[0082] The only difference between this comparative example and Example 1 is that no polyphosphate additive is added, and other conditions and parameters are exactly the same as those in Example 1.

[0083] Comparative Example 2

[0084] The only difference between this comparative example and Example 1 is that the polyphosphate additive is replaced by phosphate, and the other conditions and parameters are exactly the same as those in Example 1.

[0085] Comparative Example 3

[0086] The only difference between this comparative example and Example 1 is that all materials are added and homogenized at one time, and other conditions and parameters are exactly the same as those in Example 1.

[0087] Performance testing:

[0088] The viscosity and solid content of the positive electrode slurries prepared in the examples and comparative examples were tested at 25°C. The test results are shown in Table 1:

[0089] Table 1

[0090] As can be seen from Table 1, it can be obtained from Examples 1-3 that the positive electrode slurry prepared by the homogenization method described in the present application has a solid content of more than 69% in the viscosity range of 15000 to 18000 MPa.s. Increasing the dispersant can increase the solid content. Increasing the amount of dispersant added from 0.1% to 0.2% increases the solid content by 2 percentage points, and the viscosity does not change much. However, the addition of the dispersant has a significant effect on increasing the solid content.

[0091] By comparing Example 1 with Examples 4-5, it can be seen that in the homogenization method described in the present application, the amount of polyphosphate additive added will affect the homogenization effect. The homogenization effect is better when the mass of the polyphosphate additive is controlled at 0.05-0.2% of the mass of the solid matter in the slurry. If the amount of polyphosphate additive added is too low, the carbon layer on the surface of the lithium iron phosphate particles cannot be fully wrapped, the particles are not easy to disperse, and the effect of reducing the viscosity is not significant; if the amount of polyphosphate additive added is too high, it is easy to form a high dispersion, resulting in a low viscosity of the slurry, which is not conducive to the subsequent processing of the slurry.

[0092] From the comparison between Example 1 and Comparative Example 1, it can be seen that the present application adds a polyphosphate ester auxiliary agent during the homogenization process of the positive electrode slurry, and adopts a step-by-step addition method for homogenization. The polyphosphate ester auxiliary agent contains an anchoring group and a solvation chain. The anchoring group of the polyphosphate ester auxiliary agent is a group containing phosphoric acid, which preferentially combines with the carbon layer on the surface of the lithium iron phosphate, playing a very good anchoring role.

[0093] From the comparison between Example 1 and Comparative Example 2, it can be seen that phosphate ester has little effect on improving viscosity. Since phosphate ester substances are single molecules, while polyphosphate esters are high molecular weight compounds with longer solvation chains, they can increase the mutual repulsion between particles and have a good stabilizing effect on the dispersion of particles. The dispersion effect is more effective than that of single molecule phosphate.

[0094] From the comparison between Example 1 and Comparative Example 3, it can be seen that the viscosity of the slurry obtained by one-time homogenization is the highest. Since the components in the slurry cannot be fully and evenly dispersed during the one-time homogenization process, more agglomerates are easily formed. The addition of a dispersing aid can effectively avoid the occurrence of agglomeration between particles, forming a well-dispersed and uniform slurry. The step-by-step homogenization process is significantly better than the one-time homogenization process.

Claims

1. A method for homogenizing a positive electrode slurry, the method comprising the following steps: The first part of the conductive agent, the binder and the polyphosphate additive are mixed with the first part of the solvent, and a first mixed material is obtained by stirring in one step; The first part of the lithium iron phosphate positive electrode material and the second part of the conductive agent are mixed with the first mixed material, and a second mixed material is obtained by two-step stirring; After the second part of lithium iron phosphate positive electrode material and the second mixed material are evenly mixed, the second part of solvent is added, and the positive electrode slurry is obtained through three steps of stirring.

2. The homogenization method according to claim 1, wherein: The first part of the conductive agent and the second part of the conductive agent constitute a conductive agent, the first part of the lithium iron phosphate positive electrode material and the second part of the lithium iron phosphate positive electrode material constitute a lithium iron phosphate positive electrode material, and the first part of the solvent and the second part of the solvent constitute a solvent.

3. The homogenization method according to claim 2, wherein: The first part of the conductive agent includes any one of conductive carbon black, carbon nanotubes or graphene or a combination of at least two thereof; the second part of the conductive agent includes any one of conductive carbon black, carbon nanotubes or graphene or a combination of at least two thereof; Preferably, the mass of the first part of the conductive agent accounts for 20-60% of the total mass of the conductive agent; Preferably, the binder comprises polyvinylidene fluoride; Preferably, the first part of the solvent comprises N-methylpyrrolidone, and the second part of the solvent comprises N-methylpyrrolidone; Preferably, the mass of the first part of the solvent accounts for 80-90% of the total mass of the solvent.

4. The homogenization method according to any one of claims 1 to 3, wherein: In the step of mixing the first part of the conductive agent, the binder and the polyphosphate additive with the first part of the solvent, and stirring in one step to obtain a first mixed material, the stirring speed in the one step is 1150-1250 rpm; Preferably, the stirring time of the step is 15 to 25 minutes.

5. The homogenization method according to any one of claims 1 to 4, wherein: In the step of mixing the first part of lithium iron phosphate positive electrode material and the second part of conductive agent with the first mixed material to obtain the second mixed material through two steps of stirring, the mass of the first part of lithium iron phosphate positive electrode material accounts for 40-60% of the total mass of the lithium iron phosphate positive electrode material; Preferably, the mass of the second portion of the conductive agent accounts for 40-80% of the total mass of the conductive agent.

6. The homogenization method according to any one of claims 1 to 4, wherein: In the step of mixing the first part of lithium iron phosphate positive electrode material and the second part of conductive agent with the first mixed material, and obtaining the second mixed material through two-step stirring, the speed of the two-step stirring is 750-850 rpm; Preferably, the second-step stirring time is 8 to 12 minutes.

7. The homogenization method according to any one of claims 1 to 6, wherein: After the second part of the lithium iron phosphate positive electrode material and the second mixed material are evenly mixed, the second part of the solvent is added, and in the step of obtaining the positive electrode slurry through three steps of stirring, the mass of the second part of the lithium iron phosphate positive electrode material accounts for 40-60% of the total mass of the lithium iron phosphate positive electrode material; Preferably, the mass of the second portion of solvent accounts for 10-20% of the total mass of the solvent.

8. The homogenization method according to any one of claims 1 to 7, wherein: After the second part of the lithium iron phosphate positive electrode material and the second mixed material are evenly mixed, the second part of the solvent is added, and the positive electrode slurry is obtained by three-step stirring, wherein the stirring speed of the three steps is 500-600 rpm; Preferably, the vacuum degree of the three-step stirring is -0.09 to -0.08 MPa; Preferably, the three-step stirring time is 3 to 6 hours.

9. A positive electrode slurry, the positive electrode slurry being prepared by the homogenization method according to any one of claims 1 to 8, the positive electrode slurry comprising a solid substance and a solvent; Preferably, the solid material comprises lithium iron phosphate positive electrode material, a binder, a conductive agent and a polyphosphate additive; Preferably, based on the mass of the solid substance being 100%, the mass fraction of the lithium iron phosphate positive electrode material is 95-99%; Preferably, based on the mass of the solid substance being 100%, the mass fraction of the conductive agent is 0.3-0.5%; Preferably, based on the mass of the solid material being 100%, the mass fraction of the binder is 1 to 3%; Preferably, based on the mass of the solid substance being 100%, the mass fraction of the polyphosphate additive is 0.05 to 0.2%; Preferably, based on 100% of the mass of the positive electrode slurry, the mass fraction of the solvent is 30-50%; Preferably, the viscosity of the positive electrode slurry is 8000-30000 mPa.s; Preferably, the solid content of the positive electrode slurry is 65-70%.

10. A positive electrode sheet, which is obtained by coating the positive electrode slurry according to any one of claims 1 to 9 on the surface of a positive electrode current collector, followed by drying and roller pressing. 11 . A lithium ion battery, comprising the positive electrode sheet according to claim 10 .

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