A positive electrode slurry, a method for homogenizing the same, and use thereof

Abstract

This invention provides a positive electrode slurry, its homogenization method, and its application. The homogenization method includes the following steps: (1) mixing a first portion of conductive agent, binder, and polyphosphate additive with a first portion of solvent, and stirring in one step to obtain a first mixture; (2) mixing a first portion of lithium iron phosphate positive electrode material and a second portion of conductive agent with the first mixture, and stirring in two steps to obtain a second mixture; (3) mixing the second portion of lithium iron phosphate positive electrode material with the second mixture until uniform, adding a second portion of solvent, and stirring in three steps to obtain the positive electrode slurry. The homogenization method of this invention is simple in process, has low equipment requirements, and produces a slurry with good dispersibility and high solid content, which can greatly improve production efficiency, while saving solvent usage and reducing production costs.

Description

Technical Field

[0001] This invention belongs to the field of lithium-ion battery technology, and relates to a positive electrode slurry, its homogenization method, and its application. Background Technology

[0002] Existing conventional lithium battery slurries typically contain active materials, binders, conductive agents, and solvents. Due to the large proportion of active materials in the slurry, properties such as the solid content, viscosity changes, and sedimentation are greatly affected by the active materials.

[0003] The preparation process of lithium-ion battery electrodes includes steps such as slurry preparation, coating, rolling, and cutting. The uniformity of the dispersion of each component in the battery slurry directly affects the processing performance of the electrode, with the slurry preparation process accounting for more than 30% of the impact on battery performance. To improve slurry preparation efficiency, the active materials are usually modified. To adapt to different active materials, corresponding slurry preparation processes need to be developed, which seriously affects production efficiency.

[0004] Lithium iron phosphate is usually coated with a carbon layer. The surface of the coated carbon layer contains abundant oxygen-containing functional groups. These oxygen-containing functional groups can easily form hydrogen bonds with materials such as PVDF, thus 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, which increases the production cost.

[0005] CN114156431A discloses a positive electrode homogenization method, a positive electrode slurry, and their applications. The positive electrode homogenization method includes the following steps: 1) mixing a conductive agent, a first binder, and a positive electrode active material into a clump; 2) mixing a PVP dispersion with the clump to obtain a first dispersion; 3) homogenizing the slurry using the first dispersion, the conductive agent, and a second binder to obtain a positive electrode slurry; the first binder includes at least one of polyimide and modified polyimide.

[0006] CN103022420A discloses a method for homogenizing the positive electrode of a lithium-ion power battery to improve the uniformity of the electrode slurry, comprising the following steps: placing a dry powder conductive agent and a diluted binder into a homogenizer with a circulating water cooling function and stirring at low speed 20 rpm and high speed 2000 rpm simultaneously for 30 min; adding a paste-like conductive agent and N-methylpyrrolidone and stirring at low speed 20 rpm and high speed 2000 rpm simultaneously for 60 min; adding active material and stirring at low speed 20 rpm for 10 min, and then stirring at low speed 20 rpm and high speed 2000 rpm simultaneously for 60 min.

[0007] The homogenization method described above is relatively complex and requires high equipment operating power, resulting in a low solid content and poor dispersibility of the slurry. Summary of the Invention

[0008] The purpose of this invention is to provide a positive electrode slurry, its homogenization method, and its application. The homogenization method of this invention is simple in process, has low equipment requirements, and produces a slurry with good dispersibility and high solid content, which can greatly improve production efficiency, while saving solvent usage and reducing production costs.

[0009] To achieve this objective, the present invention adopts the following technical solution:

[0010] In a first aspect, the present invention provides a method for homogenizing a positive electrode slurry, the homogenization method comprising the following steps:

[0011] (1) The first part of conductive agent, binder and polyphosphate ester additive are mixed with the first part of solvent and stirred in one step to obtain the first mixture;

[0012] (2) The first part of lithium iron phosphate cathode material and the second part of conductive agent are mixed with the first mixture, and the second mixture is obtained by two-step stirring.

[0013] (3) After the second part of lithium iron phosphate cathode material is mixed evenly with the second mixture, the second part of solvent is added, and the cathode slurry is obtained by stirring in three steps.

[0014] The one-step stirring, two-step stirring, and three-step stirring described in this invention are respectively the first stirring, the second stirring, and the third stirring.

[0015] In this invention, a polyphosphate ester additive is added during the homogenization process of the positive electrode slurry, and homogenization is carried out by step feeding. The polyphosphate ester additive contains anchoring groups and solvation chains. The anchoring groups of the polyphosphate ester additive are groups containing phosphoric acid, which preferentially bind to the carbon layer on the surface of lithium iron phosphate, thus playing a very good anchoring role.

[0016] In step (1) of this invention, conductive agent, binder, polyphosphate ester additive and part of solvent are mixed in advance to prepare conductive adhesive liquid (first mixture material), so that polyphosphate ester additive is uniformly dispersed. In step (2) during the stirring process, the phosphate group in polyphosphate ester additive takes priority over binder as anchoring group and is firmly combined with carbon layer on the surface of lithium iron phosphate in slurry, lithium iron phosphate and other substances, forming a coating layer on the particle surface, while the remaining solvation chain remains in the solvent. Due to the mutual repulsion between the same solvation chain, the stability between solid phase particles is improved.

[0017] Preferably, the conductive agent in step (1) includes any one or a combination of at least two of conductive carbon black, carbon nanotubes, or graphene.

[0018] Preferably, 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%.

[0019] Preferably, the adhesive comprises polyvinylidene fluoride.

[0020] Preferably, the solvent comprises N-methylpyrrolidone.

[0021] Preferably, 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%, etc.

[0022] Preferably, the stirring speed in step (1) is 1150 to 1250 rpm, for example: 1150 rpm, 1180 rpm, 1200 rpm, 1220 rpm or 1250 rpm.

[0023] Preferably, the stirring time for the first step is 15 to 25 minutes, for example: 15 minutes, 18 minutes, 20 minutes, 22 minutes or 25 minutes.

[0024] Preferably, in step (2), the mass of the first portion of lithium iron phosphate cathode material accounts for 40% to 60% of the total mass of lithium iron phosphate cathode material, for example: 40%, 45%, 50%, 55% or 60%, etc.

[0025] Preferably, 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%, etc.

[0026] Preferably, the stirring speed in step (2) is 750-850 rpm, for example: 750 rpm, 780 rpm, 800 rpm, 820 rpm or 850 rpm.

[0027] Preferably, the two-step stirring time is 8 to 12 minutes, for example: 8 minutes, 9 minutes, 10 minutes, 11 minutes or 12 minutes.

[0028] Preferably, in step (3), the mass of the second part of the lithium iron phosphate cathode material accounts for a certain percentage of the total mass of the lithium iron phosphate cathode material, for example: 40%, 45%, 50%, 55% or 60%, etc.

[0029] Preferably, 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%, etc.

[0030] Preferably, the stirring speed of the three steps in step (3) is 500 to 600 rpm, for example: 500 rpm, 520 rpm, 550 rpm, 580 rpm or 600 rpm.

[0031] Preferably, the vacuum degree of the three-step stirring is -0.09 to -0.08 MPa, for example: -0.08 MPa, -0.088 MPa, -0.085 MPa, -0.082 MPa or -0.08 MPa, etc.

[0032] Preferably, the stirring time for the three steps is 3 to 6 hours, for example: 3 hours, 3.5 hours, 4 hours, 5 hours or 6 hours.

[0033] In a second aspect, the present invention provides a positive electrode slurry, wherein the positive electrode sheet is prepared by the homogenization method described in the first aspect, and the positive electrode slurry comprises a solid substance and a solvent.

[0034] Preferably, the solid material includes lithium iron phosphate cathode material, binder, conductive agent and polyphosphate additive.

[0035] Preferably, the mass fraction of the lithium iron phosphate cathode material is 95-99%, for example, 95%, 96%, 97%, 98%, or 99%, based on the mass of the solid material as 100%.

[0036] Preferably, the mass fraction of the conductive agent is 0.3% to 0.5% based on the mass of the solid substance (100%), for example: 0.3%, 0.35%, 0.4%, 0.45%, or 0.5%.

[0037] Preferably, the mass fraction of the adhesive is 1-3% based on the mass of the solid substance being 100%, for example: 1%, 1.5%, 2%, 2.5% or 3%, etc.

[0038] Preferably, the mass fraction of the polyphosphate ester additive is 0.05-0.2% based on the mass of the solid substance as 100%, for example: 0.05%, 0.08%, 0.1%, 0.15% or 0.2%, etc.

[0039] Preferably, based on the mass of the positive electrode slurry as 100%, the mass fraction of the solvent is 30-50%, for example: 30%, 35%, 40%, 45% or 50%, etc.

[0040] Preferably, 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, etc.

[0041] Preferably, the solid content of the positive electrode slurry is 65-70%, for example: 65%, 66%, 67%, 68% or 70%, etc.

[0042] Thirdly, the present invention provides a positive electrode sheet, which is obtained by coating the positive electrode slurry described in the second aspect onto the surface of the positive electrode current collector, and then drying and rolling it.

[0043] Fourthly, the present invention provides a lithium-ion battery comprising a positive electrode as described in the third aspect.

[0044] Compared with the prior art, the present invention has the following beneficial effects:

[0045] (1) Under the same stirring conditions, the slurry output solid content of the homogenization method of the present invention is higher, which improves the production efficiency and reduces the amount of solvent used, greatly reducing the production cost. The solid content of the slurry is about 7% to 10% higher than that of conventional slurries, while the viscosity is the same as that of conventional slurries, without affecting the subsequent processing performance of the slurry.

[0046] (2) In the process of homogenization, the present invention adds a polyphosphate ester additive. The polyphosphate ester additive has the same phosphate group as lithium iron phosphate, which has a stronger binding ability with particles and a better dispersing ability. It can effectively disperse the positive electrode particles and improve the uniformity and stability of the slurry.

[0047] (3) The positive electrode slurry prepared by the homogenization method of the present invention has a solid content of more than 69% in the viscosity range of 15000-18000 mpa.s. Increasing the dispersant can increase the solid content. When the amount of dispersant added is increased from 0.1% to 0.2%, the solid content increases by 2 percentage points. The viscosity does not change much, but the addition of dispersant has a significant effect on increasing the solid content. Detailed Implementation

[0048] The technical solution of the present invention will be further illustrated below through specific embodiments. Those skilled in the art should understand that the embodiments described are merely illustrative of the present invention and should not be construed as limiting the invention.

[0049] Unless otherwise specified, all ratios described in the embodiments and comparative examples of this invention are mass ratios.

[0050] Example 1

[0051] This embodiment provides a homogenization method for positive electrode slurry, the homogenization method comprising the following steps:

[0052] (1) Mix 40% conductive carbon black, polyvinylidene fluoride (PVDF) and polyphosphate ester additive with 85% NMP and stir at 1200 rpm for 20 min to obtain the first mixture.

[0053] (2) Mix 50% lithium iron phosphate cathode material and the remaining 60% conductive carbon black with the first mixture, and stir at 800 rpm for 10 min to obtain the second mixture;

[0054] (3) After mixing the remaining 50% lithium iron phosphate cathode material with the second mixture, add the remaining 15% NMP and stir at 550 rpm for 4 hours at -0.085 MPa to obtain the cathode slurry. The cathode slurry includes solid matter and solvent. The mass fraction of the solvent is 30%. In the solid matter, the mass fraction of lithium iron phosphate cathode material is 96.5%, the mass fraction of conductive carbon black is 0.4%, the mass fraction of PVDF is 3%, and the mass fraction of polyphosphate ester additive is 0.1%.

[0055] Example 2

[0056] This embodiment provides a homogenization method for positive electrode slurry, the homogenization method comprising the following steps:

[0057] (1) Mix 20% conductive carbon black, polyvinylidene fluoride (PVDF) and polyphosphate ester additive with 80% NMP and stir at 1150 rpm for 25 min to obtain the first mixture.

[0058] (2) Mix 40% lithium iron phosphate cathode material and the remaining 80% conductive carbon black with the first mixture, and stir at 750 rpm for 12 min to obtain the second mixture;

[0059] (3) After mixing the remaining 60% lithium iron phosphate cathode material with the second mixture, add the remaining 20% ​​NMP and stir at 500 rpm for 6 hours at -0.08 MPa to obtain the cathode slurry. The cathode slurry includes solid matter and solvent. The mass fraction of the solvent is 31%. In the solid matter, the mass fraction of lithium iron phosphate cathode material is 98%, the mass fraction of conductive carbon black is 0.4%, the mass fraction of PVDF is 1.5%, and the mass fraction of polyphosphate ester additive is 0.1%.

[0060] Example 3

[0061] This embodiment provides a homogenization method for positive electrode slurry, the homogenization method comprising the following steps:

[0062] (1) Mix 60% conductive carbon black, polyvinylidene fluoride (PVDF) and polyphosphate ester additive with 90% NMP and stir at 1250 rpm for 15 min to obtain the first mixture.

[0063] (2) Mix 60% lithium iron phosphate cathode material and the remaining 40% conductive carbon black with the first mixture, and stir at 750 rpm for 12 min to obtain the second mixture;

[0064] (3) After mixing the remaining 40% lithium iron phosphate cathode material with the second mixture, add the remaining 10% NMP and stir at 600 rpm for 3 hours at -0.09 MPa to obtain the cathode slurry. The cathode slurry includes solid matter and solvent. The mass fraction of the solvent is 28%. In the solid matter, the mass fraction of lithium iron phosphate cathode material is 97.5%, the mass fraction of conductive carbon black is 0.3%, the mass fraction of PVDF is 2%, and the mass fraction of polyphosphate ester additive is 0.2%.

[0065] Example 4

[0066] The only difference between this embodiment and Embodiment 1 is that the mass fraction of polyphosphate ester additive in the solid matter of the slurry is 0.01%, and the mass fraction of lithium iron phosphate cathode material is 96.59%. All other conditions and parameters are exactly the same as in Embodiment 1.

[0067] Example 5

[0068] The only difference between this embodiment and Embodiment 1 is that the mass fraction of polyphosphate ester additive in the solid matter of the slurry is 0.3%, and the mass fraction of lithium iron phosphate cathode material is 96.3%. All other conditions and parameters are exactly the same as in Embodiment 1.

[0069] Comparative Example 1

[0070] The only difference between this comparative example and Example 1 is that no polyphosphate ester additive is added; all other conditions and parameters are exactly the same as in Example 1.

[0071] Comparative Example 2

[0072] The only difference between this comparative example and Example 1 is that the polyphosphate ester additive is replaced with phosphate ester; all other conditions and parameters are exactly the same as in Example 1.

[0073] Comparative Example 3

[0074] The only difference between this comparative example and Example 1 is that all materials were added and homogenized at once, while the other conditions and parameters were exactly the same as in Example 1.

[0075] Performance testing:

[0076] 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.

[0077] Table 1

[0078]

[0079]

[0080] As can be seen from Table 1, and from Examples 1-3, the positive electrode slurry prepared by the homogenization method of the present invention can achieve a solid content of over 69% within a viscosity range of 15000-18000 mPa·s. Increasing the amount of dispersant can increase the solid content. When the amount of dispersant added is increased from 0.1% to 0.2%, the solid content increases by 2 percentage points. The viscosity does not change much, but the addition of dispersant has a significant effect on increasing the solid content.

[0081] A comparison of Examples 1 and 4-5 shows that the amount of polyphosphate additive added in the homogenization method of the present invention affects the homogenization effect. Controlling the mass of the polyphosphate additive to 0.05-0.2% of the solid matter mass in the slurry results in a better homogenization effect. If the amount of polyphosphate additive added is too low, it cannot adequately coat the carbon layer on the surface of the lithium iron phosphate particles, making it difficult for the particles to disperse and resulting in an insignificant viscosity reduction effect. If the amount of polyphosphate additive added is too high, it easily leads to high dispersion, resulting in a low slurry viscosity, which is detrimental to subsequent processing of the slurry.

[0082] As can be seen from the comparison between Example 1 and Comparative Example 1, the present invention adds a polyphosphate ester additive during the homogenization process of the positive electrode slurry and adopts a step-by-step feeding method for homogenization. The polyphosphate ester additive contains anchoring groups and solvation chains. The anchoring groups of the polyphosphate ester additive are groups containing phosphoric acid, which preferentially bind to the carbon layer on the surface of lithium iron phosphate, thus playing a very good anchoring role.

[0083] As can be seen from the comparison between Example 1 and Comparative Example 2, the effect of phosphate ester on improving viscosity is minimal. Since phosphate ester is a single molecule, while polyphosphate ester is a polymer compound with a longer solvation chain, it can increase the mutual repulsion between particles and play a good stabilizing role in particle dispersion. The dispersion effect is more effective than that of single phosphate ester.

[0084] Comparing Example 1 and Comparative Example 3, it can be seen that the slurry with one-time homogenization has the highest viscosity. Because the components in the slurry cannot be fully and uniformly dispersed during the one-time homogenization process, it is easy to form more agglomerates. The addition of dispersing agent can effectively avoid the occurrence of agglomeration between particles and form a slurry with good dispersibility and uniformity. The step-by-step homogenization process is significantly better than the one-time homogenization process.

[0085] The applicant declares that the above description is only a specific embodiment of the present invention, but the protection scope of the present invention is not limited thereto. Those skilled in the art should understand that any changes or substitutions that can be easily conceived by those skilled in the art within the technical scope disclosed in the present invention fall within the protection and disclosure scope of the present invention.

Claims

1. A method of homogenizing a positive electrode slurry, characterized by, The homogenization method includes the following steps: (1) The first part of conductive agent, binder and polyphosphate ester additive are mixed with the first part of solvent and stirred in one step to obtain the first mixture; (2) The first part of lithium iron phosphate cathode material and the second part of conductive agent are mixed with the first mixture and stirred in two steps to obtain the second mixture; (3) After the second part of lithium iron phosphate cathode material is mixed evenly with the second mixture, the second part of solvent is added, and the cathode slurry is obtained by stirring in three steps; Both the first part of the lithium iron phosphate cathode material and the second part of the lithium iron phosphate cathode material are lithium iron phosphate cathode materials with a carbon layer on the surface.

2. The homogenization method of claim 1, wherein, The conductive agent in step (1) includes any one or a combination of at least two of conductive carbon black, carbon nanotubes or graphene.

3. The homogenization method of claim 1, wherein, The first portion of the conductive agent accounts for 20-60% of the total mass of the conductive agent.

4. The homogenization method of claim 1, wherein, The adhesive includes polyvinylidene fluoride.

5. The homogenization method as described in claim 1, characterized in that, The solvent includes N-methylpyrrolidone.

6. The homogenization method as described in claim 1, characterized in that, The mass of the first portion of solvent accounts for 80-90% of the total mass of the solvent.

7. The homogenization method as described in claim 1, characterized in that, The stirring speed in step (1) is 1150~1250 rpm.

8. The homogenization method as described in claim 7, characterized in that, The stirring time for this step is 15-25 minutes.

9. The homogenization method as described in claim 1, characterized in that, Step (2) The mass of the first part of the lithium iron phosphate cathode material accounts for 40-60% of the total mass of the lithium iron phosphate cathode material.

10. The homogenization method according to claim 1, characterized in that, The second part of the conductive agent accounts for 40-80% of the total mass of the conductive agent.

11. The homogenization method as described in claim 1, characterized in that, The stirring speed in step (2) is 750~850 rpm.

12. The homogenization method as described in claim 11, characterized in that, The two-step stirring time is 8-12 minutes.

13. The homogenization method as described in claim 1, characterized in that, In step (3), the mass of the second part of the lithium iron phosphate cathode material accounts for 40-60% of the total mass of the lithium iron phosphate cathode material.

14. The homogenization method as described in claim 1, characterized in that, The second part of the solvent accounts for 10-20% of the total solvent mass.

15. The homogenization method according to claim 1, characterized in that, The stirring speed in step (3) is 500~600 rpm.

16. The homogenization method as described in claim 15, characterized in that, The vacuum degree of the three-step stirring is -0.09 to -0.08 MPa.

17. The homogenization method as described in claim 15, characterized in that, The stirring time for the three steps is 3 to 6 hours.

18. A positive electrode slurry, characterized in that, The positive electrode slurry is prepared by the homogenization method according to any one of claims 1-17, wherein the positive electrode slurry comprises a solid substance and a solvent.

19. The positive electrode slurry as described in claim 18, characterized in that, The solid material includes lithium iron phosphate cathode material, binder, conductive agent and polyphosphate additive.

20. The positive electrode slurry as described in claim 19, characterized in that, Based on the mass of the solid material being 100%, the mass fraction of the lithium iron phosphate cathode material is 95-99%.

21. The positive electrode slurry as described in claim 19, characterized in that, Based on the mass of the solid substance being 100%, the mass fraction of the conductive agent is 0.3~0.5%.

22. The positive electrode slurry as described in claim 19, characterized in that, The mass fraction of the binder is 1-3%, based on the mass of the solid substance being 100%.

23. The positive electrode slurry as described in claim 19, characterized in that, Based on the mass of the solid substance as 100%, the mass fraction of the polyphosphate ester additive is 0.05~0.2%.

24. The positive electrode slurry as described in claim 18, characterized in that, Based on the mass of the positive electrode slurry being 100%, the mass fraction of the solvent is 30-50%.

25. The positive electrode slurry as described in claim 18, characterized in that, The viscosity of the positive electrode slurry is 8000~30000 mPa·s.

26. The positive electrode slurry as described in claim 18, characterized in that, The solid content of the positive electrode slurry is 65-70%.

27. A positive electrode sheet, characterized in that, The positive electrode sheet is obtained by coating the positive electrode slurry as described in any one of claims 18-26 onto the surface of the positive electrode current collector, followed by drying and roller pressing.

28. A lithium-ion battery, characterized in that, The lithium-ion battery includes the positive electrode as described in claim 27.

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