A positive electrode slurry, a method for preparing the same, and an application thereof

Abstract

This invention discloses a positive electrode slurry, its preparation method, and its application, relating to the field of lithium-ion secondary battery technology. The invention uses polyvinylidene fluoride (PVDF) with a certain degree of branching as a binder. By controlling the content of long and short branched chains, the stability of the positive electrode slurry can be significantly improved, which is beneficial for the coating of the positive electrode sheet and ensures the continuity of production. Simultaneously, the obtained positive electrode slurry has strong adhesion, which can improve the peel strength of the positive electrode sheet. Furthermore, due to the good stability of the positive electrode slurry described in this invention, the areal density of the obtained positive electrode sheet is stable, and the prepared battery exhibits good consistency. The viscosity of the positive electrode slurry described in this invention does not exceed 58.8% after standing for 48 hours, demonstrating good stability, and the peel strength of the prepared positive electrode sheet is greater than 13.7 N / m.

Description

Technical Field

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

[0002] With the implementation of the dual-carbon policy, the new energy vehicle industry has developed rapidly, driving the development of lithium-ion power batteries. In the lithium battery manufacturing process, the stability of the lithium battery slurry is one of the important indicators of its properties. The stability of the slurry affects its storage time; the more stable the slurry, the more continuous the production process can be. At the same time, the stability of the slurry also has a significant impact on the coating effect. The better the slurry stability, the more stable the coating process, the more stable the electrode surface density, and the better the consistency of the prepared battery. However, currently commonly used lithium battery slurries have poor stability. This is because lithium battery slurries generally use polyvinylidene fluoride (PVDF) as a binder, and the interaction between the PVDF molecular chains and LFP (lithium iron phosphate) particles is unstable, resulting in poor slurry stability.

[0003] The prior art discloses a composite binder for lithium-sulfur batteries, which is composed of PVDF and PU. The PU is a branched polyester polyurethane. The addition of PU reduces the regularity of PVDF, weakens the crystallization tendency of PVDF, and improves the stability of the binder. However, it does not solve the problem of unstable slurry caused by the unstable interaction between the binder and the active material LFP. Summary of the Invention

[0004] In order to overcome the poor stability of existing lithium-ion battery cathode slurries using LFP as the active material, this invention provides a cathode slurry that, by controlling the degree of branching and the content of long branched chains of polyvinylidene fluoride, enables the obtained polyvinylidene fluoride binder to fully contact the LFP active material, forming a stable interaction and improving the stability of the cathode slurry.

[0005] Another object of the present invention is to provide a positive electrode sheet prepared from the above-mentioned positive electrode slurry.

[0006] Another object of the present invention is to provide an application of the above-mentioned positive electrode sheet in a lithium-ion secondary battery.

[0007] Another object of the present invention is to provide a lithium-ion secondary battery prepared with the above-mentioned positive electrode sheet.

[0008] The above-mentioned objective of this invention is achieved through the following technical solution:

[0009] A positive electrode slurry includes a solid substance and a solvent, wherein the solid substance includes an active substance, a binder, and a conductive agent, the active substance being LFP, and the binder being polyvinylidene fluoride with a branching degree of 2 to 15‰.

[0010] It should be noted that:

[0011] This invention uses polyvinylidene fluoride with a certain degree of branching as a binder, which can significantly improve the stability of the positive electrode slurry.

[0012] The stability of a slurry refers to whether its viscosity and solid content (solid content) change significantly after a certain period of time. Generally, slurries with a viscosity increase of less than 40% after 24 hours of standing and less than 60% after 48 hours are considered to have good stability and are acceptable in production. Changes in the solid content of the slurry can also reflect its stability to some extent, usually expressed as changes in the upper layer solid content. When the slurry is fluid, if the slurry system is stable, the upper layer solid content remains unchanged. If the slurry system is unstable, the active material settles, and the upper layer solid content decreases. If the slurry gels, although the upper layer solid content may not change significantly, the slurry system lacks fluidity and cannot be coated; therefore, the slurry is unstable.

[0013] The particle size of the active material LFP is usually 0.5 to 1.5 μm, and its surface is coated with a certain amount of carbon. It has poor dispersibility in NMP (N-methylpyrrolidone) solvent. However, polyvinylidene fluoride with a certain degree of branching can provide sufficient contact points to contact LFP, avoid the agglomeration between LFP particles, improve the dispersibility of LFP in solvent, and establish stable interaction, thereby improving the stability of the positive electrode slurry. When the branching degree of polyvinylidene fluoride (PVDF) molecular chains is low, there are too few contact points between PVDF and LFP particles, resulting in poor slurry stability and easy agglomeration of LFP particles, making it impossible to improve dispersibility. In addition, excessively low PVDF branching also reduces the adhesion of the binder. Conversely, when the branching degree of PVDF molecular chains is high, interactions easily occur between PVDF molecular chains, leading to increased slurry viscosity and gelation. The resulting cathode slurry not only has poor stability but also poor flowability, making it difficult to coat the cathode sheet smoothly. During the production process, poor slurry stability can easily lead to gelation in the transmission pipeline, affecting production.

[0014] The degree of branching mentioned in this invention refers to the percentage of carbon atoms with branched chains on the main chain of a polymer. When N out of 1000 carbon atoms have branched chains, the degree of branching is N‰.

[0015] In vinylidene fluoride polymers, long branched chains refer to branches with side chains of -CH2CF2-CH(CH2CF2-)CF2-; short branched chains refer to branches with structures of HO-CH2CF2CH2-, -CF2CF2CH3, -CF2CF2CH3, or CH2CF2CH2CF2H; the specific degree of branching can be calculated by NMR fluorine spectroscopy.

[0016] Preferably, the degree of branching of the polyvinylidene fluoride is 3 to 9‰.

[0017] Preferably, the polyvinylidene fluoride contains long-branched chains and short-branched chains, with the content of long-branched chains being 1-5‰.

[0018] When the branching degree of polyvinylidene fluoride comes entirely from short branched chains, the lack of long branched chains to wrap around the positive electrode LFP particles reduces the interaction within the entire slurry system, which is not conducive to improving the stability of the slurry.

[0019] When the content of long branched chains is too high, the interaction between polyvinylidene fluoride (PVDF) molecular chains becomes more pronounced. Therefore, controlling the content of long branched chains is more conducive to obtaining a cathode slurry with a suitable viscosity. Furthermore, since long branched chains can wrap around LFP particles, the contact points with the active material increase, which can further improve the stability of the cathode slurry.

[0020] The total content of long-branched chains and short-branched chains is equal to the degree of branching.

[0021] More preferably, the content of the long branched chain is 1.2 to 3‰.

[0022] Preferably, the weight-average molecular weight of the polyvinylidene fluoride is 400,000 to 1,000,000.

[0023] The molecular weight of polyvinylidene fluoride (PVDF) also affects the stability of the cathode slurry. When the molecular weight is large, the PVDF molecular chains are long and tend to entangle with each other, which can easily lead to slurry gelation. When the molecular weight is small, the interaction between the PVDF molecular chains and LFP particles is small, and the LFP particles are more likely to settle. Therefore, controlling the molecular weight within the range of 400,000 to 1,000,000 can further improve the stability of the cathode slurry.

[0024] Preferably, the polyvinylidene fluoride has a weight-average molecular weight of 700,000 to 800,000.

[0025] Specifically, the solid material has a mass content of 55% to 75% in the positive electrode slurry.

[0026] The content of solid matter (solid content) is a common indicator in lithium battery production. Low solid content leads to low production efficiency and high energy consumption, while high solid content can affect the fluidity of the slurry.

[0027] Specifically, the mass content of the binder in the solid material is 1-4%, preferably 1-3%, and more preferably 2%.

[0028] The binder content in the cathode slurry affects the peel strength, conductivity, and active material content of the electrode. When the binder content decreases, the slurry viscosity decreases, resulting in a lower peel strength for the cathode electrode. Conversely, increasing the binder content reduces the effective contact between the active material and the conductive agent, affecting the conductivity of the cathode electrode and also lowering the active material content, leading to a decrease in battery energy density. Therefore, properly controlling the binder content can yield a cathode slurry with better performance.

[0029] Specifically, the mass content of the active substance in the solid material is 93-98%.

[0030] Specifically, the polyvinylidene fluoride is prepared by emulsion polymerization, wherein the polymerization reaction temperature is 60–120°C.

[0031] Controlling the polymerization temperature of polyvinylidene fluoride (PVDF) is beneficial for obtaining PVDF with a certain degree of branching. When the polymerization temperature is too low, the branching is too low; when the polymerization temperature is too high, the branching is too high.

[0032] Specifically, the preparation method of the polyvinylidene fluoride can refer to the following steps:

[0033] S1. Add deionized water and emulsifier to the reactor, evacuate the reactor and replace it with nitrogen until the oxygen content is less than 10 ppm, then heat the reactor to 60-120°C, fill the reactor with vinylidene fluoride to a pressure of 2.0-8.0 MPa, and add 40% of the total amount of initiator and chain transfer agent to carry out the polymerization reaction.

[0034] S2. During the reaction, the reaction pressure is maintained by adding vinylidene fluoride. When the amount of vinylidene fluoride reacts to 15%, 40%, and 75% of the total reaction, 20% of the total amount of initiator and chain transfer agent is added each time. The reaction is stopped after it is complete.

[0035] The present invention also protects a positive electrode sheet comprising the above-mentioned positive electrode slurry.

[0036] The positive electrode slurry obtained by this invention has high stability and its viscosity does not change after being stored for a long time of 48 hours, which is beneficial to the coating of the positive electrode sheet and can provide continuous production assurance. Furthermore, due to the good adhesion of the positive electrode slurry, the resulting positive electrode sheet has strong peel strength.

[0037] This invention specifically protects the application of the above-mentioned positive electrode sheet in the preparation of lithium-ion secondary batteries.

[0038] This invention particularly protects a lithium-ion secondary battery, which includes the above-mentioned positive electrode sheet.

[0039] The lithium-ion secondary battery can be assembled in the following ways:

[0040] Artificial graphite (negative electrode material), acetylene black (conductive agent), styrene-butadiene emulsion (SBR) (binder), and sodium carboxymethyl cellulose (CMC) (dispersant) were thoroughly mixed in a deionized water solvent system at a ratio of 96.2:0.8:1.2:1.8. The mixture was then coated onto copper foil (negative electrode current collector). After dry pressing, the negative electrode sheet was obtained through processes such as rolling, slitting, and welding tabs. A porous PE separator was used, and the electrolyte was a 1 mol / L LiPF6 solution. The solvents were ethylene carbonate (EC), diethyl carbonate (DEC), and ethyl methyl carbonate (EMC) in a mass ratio of 3:5:2. The positive electrode sheet, negative electrode sheet, and separator were assembled into a soft-pack battery of approximately 3 Ah (1C constant capacity) in a glove box.

[0041] Compared with the prior art, the beneficial effects of the present invention are:

[0042] This invention uses polyvinylidene fluoride (PVDF) with a certain degree of branching as a binder. By controlling the content of long and short branched chains, the stability of the cathode slurry can be significantly improved, which is beneficial for cathode sheet coating and ensures production continuity. Simultaneously, the resulting cathode slurry has strong adhesion, which can improve the peel strength of the cathode sheet. Furthermore, due to the good stability of the cathode slurry described in this invention, the resulting cathode sheet has a stable areal density, and the prepared batteries exhibit good consistency.

[0043] The positive electrode slurry of the present invention exhibits a viscosity increase of no more than 38.7% after 24 hours of storage and less than 60% after 48 hours of storage, demonstrating good stability. Furthermore, the peel strength of the prepared positive electrode sheet is greater than 13.7 N / m. Detailed Implementation

[0044] The present invention will be further described below with reference to specific embodiments, but the embodiments do not limit the present invention in any way. Unless otherwise stated, the raw materials and reagents used in the embodiments of the present invention are conventionally purchased raw materials and reagents.

[0045] The method for preparing polyvinylidene fluoride used in the embodiments and comparative examples of this invention is as follows:

[0046] S1. In a 150L autoclave, add 105kg of deionized water and 36g of sodium perfluoropolyether carboxylate emulsifier. Vacuum the autoclave with nitrogen until the oxygen content is less than 10ppm. Heat the autoclave to 60–120℃, and charge it with polyvinylidene fluoride to a pressure of 2.0–8.0MPa. Add the initial initiator ammonium persulfate and the chain transfer agent diethyl malonate to initiate the polymerization reaction. The initial initiator and chain transfer agent account for 40% of the total initiator and chain transfer agent. The reaction temperature and pressure are adjusted according to the degree of branching and weight-average molecular weight of polyvinylidene fluoride in each example and comparative example; the amount of chain transfer agent is adjusted according to the weight-average molecular weight of polyvinylidene fluoride.

[0047] S2. Continuously replenish vinylidene fluoride to maintain constant pressure in the reactor. When the amount of vinylidene fluoride reacted is 15%, 40%, and 75% of the total reaction, 20% of initiator and chain transfer agent are added respectively. When the amount of reaction reaches 36 kg, the reaction is stopped, and then the reactor is depressurized to obtain polyvinylidene fluoride emulsion. The emulsion is filtered, washed, and dried to obtain polyvinylidene fluoride.

[0048] In this invention, the weight-average molecular weight was obtained by gel permeation chromatography (GPC), and the mobile phase was N,N-dimethylacetamide (DMAc).

[0049] Branching degree was measured using nuclear magnetic resonance (NMR). 19 F spectrum.

[0050] The testing methods for long-branched chains and short-branched chains are as follows:

[0051] Long branched chains refer to chains containing side chains of -CH2CF2-CH(CH2CF2-)CF2-. 19 At -99.5 ppm in the F spectrum; long branched chain content C=I (-99.5ppm) / I (-92ppm～118ppm) I (-99.5ppm) It refers to 19 In the F spectrum, the peak integral intensity at -99.5 ppm, I (-92ppm～118ppm) It refers to the sum of the integrated intensities of all peaks from -92ppm to 118ppm.

[0052] Short branched chains refer to all branches containing the structures HO-CH2CF2CH2-, -CF2CF2CH3, or -CH2CF2CH2CF2H; among which HO-CH2CF2CH2- is... 19 In the F spectrum, -CF2CF2CH3 is at -104.5 ppm, -CH2CF2CH2CF2H is at -114.5 ppm and 92.3 ppm, respectively; the short branch content C = [I (-104.5ppm) +I (-107.6ppm) +I (-114.5ppm)+I (-92.3ppm) ] / I (-92ppm～118ppm) .

[0053] Example 1

[0054] A positive electrode slurry comprises a solid material and a solvent, wherein the solid material includes an active material, a binder, and a conductive agent; the active material is lithium iron phosphate, and the binder is polyvinylidene fluoride with a branching degree of 8.7%; wherein the polyvinylidene fluoride contains 2.5% long-branched chains and 6.2% short-branched chains; the weight-average molecular weight of the polyvinylidene fluoride is 72W; the mass content of the active material in the solid material is 97%; the mass content of the binder is 2%; and the mass content of the solid material in the positive electrode slurry is 63.1%.

[0055] The conductive agent in the positive electrode slurry is conductive carbon black; the solvent in the positive electrode slurry is N-methylpyrrolidone.

[0056] The above-mentioned positive electrode slurry can be prepared by the following method:

[0057] The active material, conductive agent and polyvinylidene fluoride were mixed in a roller mixer for 1 hour, and then the mixture was transferred to a 5L double planetary mixer. N-methylpyrrolidone (NMP) was added and the mixture was stirred at high speed of 1500 rpm for 2 hours to synthesize a slurry.

[0058] Examples 2-5

[0059] A positive electrode slurry, with the same composition as in Example 1, with differences shown in Table 1.

[0060] Table 1. Cathode slurry parameters for Examples 1-5

[0061] Branching degree / % Long branched chain content / % Short branched chain content / % Example 1 8.7 2.5 6.2 Example 2 2.2 1.0 1.2 Example 3 3.3 1.2 2.1 Example 4 6.5 2.8 3.7 Example 5 14.9 5.0 9.9

[0062] Example 6

[0063] A positive electrode slurry with the same composition as in Example 1, except that the weight-average molecular weight of polyvinylidene fluoride is 78W.

[0064] Example 7

[0065] A positive electrode slurry with the same composition as in Example 1, except that the weight-average molecular weight of polyvinylidene fluoride is 41W.

[0066] Example 8

[0067] A positive electrode slurry with the same composition as in Example 1, except that the weight-average molecular weight of polyvinylidene fluoride is 98W.

[0068] Example 9

[0069] A positive electrode slurry, with the same composition as in Example 1, differs in that the mass content of the binder in the solid material is 1%, and the content of the active material is 98%.

[0070] Example 10

[0071] A positive electrode slurry, with the same composition as in Example 1, differs in that the mass content of the binder in the solid material is 4%, and the content of the active material is 93%.

[0072] Example 11

[0073] A positive electrode slurry, with the same composition as in Example 1, differs in that the mass content of solid matter in the positive electrode slurry is 55.4%.

[0074] Example 12

[0075] A positive electrode slurry, with the same composition as in Example 1, differs in that the mass content of solid matter in the positive electrode slurry is 74.8%.

[0076] Comparative Examples 1-2

[0077] A positive electrode slurry, with the same composition as in Example 1, with differences shown in Table 2.

[0078] Table 2. Comparative Example Cathode Slurry Parameters

[0079] Branching degree / % Long branched chain content / % Short branched chain content / % Comparative Example 1 0.5 0.2 0.3 Comparative Example 2 16.6 7.2 9.4

[0080] Comparative Example 3

[0081] A positive electrode slurry, with the same composition as in Example 1, differs in that the mass content of the binder in the solid material is 0.5%, and the content of the active material is 98%.

[0082] Comparative Example 4

[0083] A positive electrode slurry, with the same composition as in Example 1, differs in that the mass content of the binder in the solid material is 5%, and the content of the active material is 93%.

[0084] Result detection

[0085] (1) Viscosity test of positive electrode slurry

[0086] Take 120g of slurry and place it in a 150ml beaker, seal it with sealing film, and place it in a water bath at 2520.2℃ for 1 hour. Use a Bollerfeld rotational viscometer (model DV2TLVTJ0, rotor No. 63, rotation speed 12rpm) to test the viscosity, and record it as the initial viscosity η0. After the slurry has been left to stand for 24 hours and 48 hours, test its viscosity values, and record them as viscosity η1 and η2, respectively. The viscosity growth rate after 24 hours of standing = (η1-η0) / η0×100%, and the viscosity growth rate after 48 hours of standing = (η2-η0) / η0×100%.

[0087] (2) Solid content test of positive electrode slurry

[0088] Take 50g of slurry and place it in a 100ml beaker. Test the solid content of the slurry after 0h, 24h, and 48h. The method for testing the solid content is as follows: Take 1g of the upper layer of slurry from the beaker and place it on an aluminum foil tray. Then dry it in a 100℃ oven for 6h to remove the solvent from the slurry. Weigh the remaining solids. Solid content = W 干燥后 / W干燥前 ×100%.

[0089] (3) Positive electrode peel strength test

[0090] Positive electrode preparation: The prepared slurry (without standing) was uniformly coated on both sides of a 12µm thick aluminum foil using a scraper. The foil was then baked in a forced-air oven at 100℃ for 30 minutes, achieving a single-sided coating density of 160 g / m². 2 The surface density of both sides is 320 g / m³. 2 Then, it is rolled using a roller press to control the compaction density to 2.5 g / cm³. 3 To obtain the positive electrode sheet.

[0091] Peel strength test: Attach one side of 3M HVB double-sided tape (19mm*60mm) to one end of the steel plate. Then, cut the negative electrode sheet into strips of 20mm*200mm and attach the positive active layer side of each strip to the double-sided tape. Under conditions of 25℃ and 50% relative humidity, measure the stress required to peel the aluminum foil at a speed of 100mm / min in a 180° direction. This stress is the peel strength; the stronger the peel strength, the greater the adhesion.

[0092] The results are shown in Table 3.

[0093] Table 3. Test results of cathode slurry performance in each embodiment and comparative example

[0094]

[0095]

[0096] As can be seen from Table 3, the positive electrode slurries in Examples 1 to 12 have good stability. After standing for 48 hours, the viscosity increase is less than 58.8%, and the solid content of the slurry hardly changes after standing for 48 hours. Moreover, when the branching degree is high, the molecular weight of polyvinylidene fluoride is high, or the content of binder in the positive electrode slurry is high, the stability of the slurry will decrease. In Comparative Example 1, when the branching degree of polyvinylidene fluoride is too low, the obtained positive electrode slurry has low viscosity and poor adhesion. Furthermore, sedimentation occurs after standing for 48 hours, resulting in uneven density of the positive electrode coating surface. In Comparative Example 2, when the branching of polyvinylidene fluoride was too high, the interaction between molecular chains was enhanced, and the viscosity was too high after standing for 48 hours, resulting in gelation and making it impossible to coat the positive electrode sheet. In Comparative Example 3, the binder content in the positive electrode slurry was too low, resulting in poor viscosity and low peel strength of the positive electrode sheet. In Comparative Example 4, the binder content in the positive electrode slurry was too high, leading to gelation after standing for 24 hours. At this time, it was impossible to coat the positive electrode sheet smoothly, affecting the continuity of production. Moreover, due to the instability of the slurry, it was easy to gel in the pipeline, affecting production.

[0097] Obviously, the above embodiments of the present invention are merely examples for clearly illustrating the present invention, and are not intended to limit the implementation of the present invention. Those skilled in the art can make other variations or modifications based on the above description. It is neither necessary nor possible to exhaustively describe all embodiments here. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included within the scope of protection of the claims of the present invention.

Claims

1. A positive electrode slurry, comprising a solid substance and a solvent, wherein, The solid substance comprises an active substance, a binder, and a conductive agent, characterized in that the active substance is LFP, the binder is polyvinylidene fluoride with a branching degree of 2-15‰, wherein the total content of long-branched chains and short-branched chains is equal to the branching degree; the long-branched chains refer to branches containing -CH2-CF2-CH(CH2CF2-)CF2-; the short-branched chains refer to branches containing HO-CH2CF2CH2-, -CF2CF2CH3, -CF2CF2CH3, or -CH2CF2CH2CF2H structures; The polyvinylidene fluoride contains long-branched chains and short-branched chains, with the content of long-branched chains being 1-5‰. The weight-average molecular weight of the polyvinylidene fluoride is 400,000 to 1,000,000. The binder content in the solid material is 1-4% by mass.

2. The positive electrode slurry as described in claim 1, characterized in that, The degree of branching of the polyvinylidene fluoride is 3-9‰.

3. The positive electrode slurry as described in claim 1, characterized in that, The content of the long branched chain is 1.2~3‰.

4. The positive electrode slurry as described in claim 1, characterized in that, The weight-average molecular weight of the polyvinylidene fluoride is 700,000 to 800,000.

5. A positive electrode plate, characterized in that, The positive electrode sheet includes the positive electrode slurry according to any one of claims 1 to 4.

6. The application of the positive electrode sheet according to claim 5 in the preparation of a lithium-ion secondary battery.

7. A lithium-ion secondary battery, characterized in that, The lithium-ion secondary battery includes the positive electrode sheet as described in claim 5.

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