Gel-free high-stability positive electrode slurry as well as preparation method and application thereof

Abstract

The invention belongs to the field of sodium ion battery materials, and mainly relates to gel-free high-stability positive electrode slurry as well as a preparation method and application thereof. The positive electrode slurry comprises the following raw materials in parts by weight: 90-96 parts of sodium ferric sulfate, 1-4 parts of a conductive agent, 2-6 parts of a binder and 0.1-0.5 part of a combined dispersant; the combined dispersing agent comprises phosphate monoester and phosphate diester; wherein R in the ester group is one or more of dodecyl, phenyl, octyl, oleyl, polyethylene glycol and aminopropyl. A mixed system of phosphate monoester and diester is used as a dispersing agent, so that the dispersing agent has high affinity with iron ions, locks active metal ions on the surfaces of particles and blocks side reaction with a solvent, a monoester short chain is rapidly anchored to the surfaces of the particles, a diester long chain extends into the solvent to form steric hindrance, agglomeration is inhibited through a dual mechanism, and the dispersion effect is improved. The dispersity and the stability of the slurry are improved.

Description

Technical Field

[0001] The present invention belongs to the technical field of battery materials, and mainly relates to a gel-free high-stability positive electrode slurry and a preparation method and application thereof. Background Art

[0002] Among the cathode materials for sodium-ion batteries, sodium iron sulfate (NaFeSO4) has attracted much attention due to its low cost and high safety. However, its electrode slurry preparation faces severe challenges: Fe² + / Fe³ + The ions are highly active and easily absorb water or occupy the binding sites of NMP and PVDF, resulting in slurry agglomeration and a surge in viscosity. At the same time, the polar surface of the material itself has poor compatibility with organic solvents.

[0003] In order to solve the above problems, carboxylic acid reagents are currently mainly used as dispersants. For example, in the high-flow sodium iron sulfate slurry of sodium ion batteries, the fluidity regulator used is a dicarboxylic acid compound HOOC-R-COOH, which is dispersed by chemical adsorption of carboxylic acid groups with metal oxides on the surface of the active material to form M-OOC- bonds. However, it has poor dispersion effect on hydrophobic materials in the slurry, such as carbon black in the conductive agent, and is difficult to destroy π-π stacking. At the same time, it has poor thermal stability and can undergo decarboxylation at 120°C. At the same time, its acidic characteristics may corrode the current collector and increase the risk of self-discharge. Its strong adsorption will compete with PVDF for binding sites, increasing the difficulty of dispersion. In other words, traditional dispersants are difficult to effectively suppress the van der Waals force and electrostatic attraction between particles, and the possibility of slurry gelation increases, resulting in uneven porosity of the coating film layer, which directly affects the battery rate performance and cycle life. Summary of the Invention

[0004] To address the gelling issues of sodium ferric sulfate slurries and the high costs and poor adhesion associated with coatings using low-solids slurries, this paper proposes adding a phosphate monoester / phosphate diester mixture to disperse the slurry and increase coating solids. This synergistic combination of strong adsorption and ion complexation allows for precise control of surface charge, improving slurry dispersion and reducing costs.

[0005] In an embodiment of the present invention, the present invention provides a gel-free high-stability sodium ion battery positive electrode slurry, the raw materials of the positive electrode slurry include: The sodium ferric sulfate comprises 90-96 parts of sodium ferric sulfate, 1-4 parts of a conductive agent, 2-6 parts of a binder, and 0.1-0.5 parts of a combined dispersant; The combined dispersant comprises, in percentage, phosphate monoester RO-PO(OH)2 and phosphate diester (RO)2-PO(OH); wherein R is one or more of dodecyl, phenyl, octyl, oleyl, polyethylene glycol, and aminopropyl; The chemical formula is: Na xFe 1-y (SO4) z , where 1.1≤x≤1.7, 0<y≤0.1, and x, y, and z are charge balanced.

[0006] In a specific embodiment, the mass ratio of the monoester of phosphate to the diester of phosphate in the combined dispersant is 1.2:1~7:3, specifically including: any one combination of monododecyl phosphate and didodecyl phosphate, monophenyl phosphate and diphenyl phosphate, monooctyl phosphate and bis(2-ethylhexyl) phosphate, and monooleyl phosphate and dioleyl phosphate.

[0007] In a specific embodiment, the conductive agent is one or more of carbon black, acetylene black, conductive graphite, and carbon nanotubes.

[0008] In a specific embodiment, the binder is one or more of polyvinylidene fluoride, polytetrafluoroethylene, and styrene-butadiene rubber.

[0009] In a specific embodiment, the positive electrode slurry further includes a solvent, and the solvent is one or more of N-methylpyrrolidone, N,N-dimethylformamide, and dimethyl sulfoxide.

[0010] Based on the same inventive concept, the present invention also provides a method for preparing the above-mentioned gel-free high-stability sodium ion battery positive electrode slurry, comprising the following steps: S1. Drying sodium ferric sulfate, part of the conductive agent, and the binder and then mixing them to obtain a dry powder mixture; S2, mixing the dry powder mixture with a solvent and stirring to obtain a dough-like slurry; S3. Dispersing the combined dispersant with a solvent and adding it to the dough-like slurry, and adding the remaining conductive agent and solvent, mixing, stirring and dispersing to obtain a positive electrode slurry.

[0011] In a specific embodiment, a solvent is added in step S2, and the solid content of the mixed system of the dry powder mixture and the solvent is 63-73%; and a solvent is added in step S3, and the solid content of the positive electrode slurry is 45-60%.

[0012] In a specific embodiment, the viscosity of the positive electrode slurry ranges from 5000 to 8000 CP.

[0013] Based on the same inventive concept, an embodiment of the present invention further provides a positive electrode sheet comprising the above-mentioned gel-free high-stability sodium ion battery positive electrode slurry, wherein the positive electrode slurry is coated by transfer coating and slit extrusion coating to form a positive electrode sheet for a sodium ion battery.

[0014] The present invention also provides a sodium ion battery, wherein the positive electrode plate of the sodium ion battery is the positive electrode plate described above.

[0015] Compared with the prior art, the present invention has the following beneficial effects: The present invention proposes to add a mixture of phosphate monoester / phosphate diester as a dispersant for the positive electrode slurry of the sodium ferric sulfate system, wherein the phosphate group in the phosphate ester molecule is + They possess high affinity and can "lock" active metal ions on the particle surface through complexation, blocking their side reaction pathways with the solvent. Simultaneously, a mixed system of monoesters and diesters can form a gradient adsorption layer, with the short monoester chains quickly anchoring to the particle surface and the long diester chains extending into the solvent to form steric hindrance. This dual mechanism inhibits agglomeration. Secondly, the ionization properties of phosphate esters can modulate the Zeta potential of the particle surface and enhance the electrostatic repulsion effect. Compared to dicarboxylic acid dispersants, phosphate esters offer superior stability over a wide pH range, preventing the dispersion failure of carboxylic acids due to pH fluctuations and significantly improving the dispersibility and stability of the slurry. DETAILED DESCRIPTION

[0016] In order to facilitate understanding of the present invention, the following preferred embodiments provide a more comprehensive and detailed description of the present invention, but the protection scope of the present invention is not limited to the following specific embodiments.

[0017] Unless otherwise defined, all technical terms used hereinafter have the same meanings as those generally understood by those skilled in the art. The technical 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.

[0018] Unless otherwise specified, various raw materials, reagents, instruments and equipment used in the present invention can be purchased from the market or prepared by existing methods.

[0019] The present invention provides a gel-free high-stability sodium ion battery positive electrode slurry. The raw materials of the positive electrode slurry include: 90-96 parts of sodium ferric sulfate, 1-4 parts of a conductive agent, 2-6 parts of a binder, and 0.1-0.5 parts of a combined dispersant; the combined dispersant includes, in percentage, phosphate monoester RO-PO(OH)2 and phosphate diester (RO)2-PO(OH); wherein R is one or more of dodecyl, phenyl, octyl, oleyl, polyethylene glycol, and aminopropyl; the chemical formula of sodium ferric sulfate is: Na x Fe 1-y (SO4) z , where 1.1≤x≤1.7, 0<y≤0.1, and x, y, and z are charge balanced.

[0020] Furthermore, the mass ratio of the monoester to the diester in the combined dispersant is 1.2:1 to 7:3. Specific combinations include: the combined dispersant is selected from the following combinations: monododecyl phosphate and didodecyl phosphate, monophenyl phosphate and diphenyl phosphate, monooctyl phosphate and bis(2-ethylhexyl) phosphate, and monooleyl phosphate and dioleyl phosphate. The conductive agent is one or more of carbon black, acetylene black, conductive graphite, and carbon nanotubes, preferably a combination of carbon black and carbon nanotubes. The binder is one or more of polyvinylidene fluoride, polytetrafluoroethylene, and styrene-butadiene rubber. The positive electrode slurry also includes a solvent, specifically one or more of N-methylpyrrolidone, N,N-dimethylformamide, and dimethyl sulfoxide.

[0021] In order to obtain a gel-free, high-stability sodium ion battery positive electrode slurry, the preparation method provided by the present invention comprises the following steps: S1. Dry the sodium ferric sulfate, part of the conductive agent, and the binder, then mix them to obtain a dry powder mixture. The drying temperature in this process is determined by their specific characteristics. For example, the evaporation temperature of the crystal water of the sodium ferric sulfate material is greater than or equal to 175°C, so the sodium ferric sulfate positive electrode main material is baked at 200°C for 24 hours, and the conductive agent, binder, etc. are dried at 60°C for 3 hours. During the dry powder mixing process, the dry powder stacking order is 50% sodium ferric sulfate, conductive agent, binder, and 50% sodium ferric sulfate. Dry mixing is performed for 1 hour or more to obtain a dry powder mixture.

[0022] S2. Mix the dry powder mixture with a solvent and stir to obtain a dough-like slurry; the amount of solvent added in this process is such that the solid content of the dry powder mixture and the solvent mixture system is 63-73%, and place it in a stirring tank and rotate and knead for more than 2 hours to obtain a dough-like slurry.

[0023] S3. Dispersing the combined dispersant with a solvent and adding it to the dough-like slurry, then adding the remaining conductive agent and solvent, mixing and dispersing to obtain a positive electrode slurry. The amount of solvent added during this process is such that the solids content of the slurry is between 45% and 60%. The slurry is rotated at a high speed, with a linear dispersion speed of 20 to 30 m / s. The viscosity of the positive electrode slurry obtained after stirring is in the range of 5000 to 8000 cp.

[0024] Based on the same inventive concept, the present invention further provides a positive electrode sheet comprising the aforementioned gel-free, high-stability sodium-ion battery positive electrode slurry. During the preparation process, the positive electrode slurry is applied using transfer coating or slot extrusion coating to form the sodium-ion battery positive electrode sheet. The present invention also provides a sodium-ion battery, wherein the positive electrode sheet of the sodium-ion battery is the aforementioned positive electrode sheet.

[0025] The following is further described with reference to specific examples.

[0026] Example 1 This embodiment provides a gel-free high-stability sodium ion battery positive electrode slurry raw material and a preparation method, comprising the following steps: The weight ratio of the slurry raw materials is sodium ferric sulfate: carbon black (SP): carbon nanotubes (CNT): polyvinylidene fluoride (PVDF): combined dispersant = 94:1.5:0.2:4:0.3, the solvent is N-methylpyrrolidone; the combined dispersant is a mixture of monododecyl phosphate and didodecyl phosphate, and the mass ratio of the two is: 7:3.

[0027] The preparation method comprises the following steps: The sodium ferric sulfate cathode material was baked in a 200°C oven for 24 hours, while SP and PVDF were baked at 60°C for 3 hours. After the materials cooled to room temperature, the sodium ferric sulfate, binder PVDF, and conductive agent SP were dry-mixed for 1 hour. The dry powder stacking order was 50% sodium ferric sulfate, SP, PVDF, and 50% sodium ferric sulfate, resulting in a uniform dry powder mixture.

[0028] The dry powder mixture was added into a stirring tank, and the solvent N-methylpyrrolidone was added to make the solid content of the dry powder mixture and the solvent mixed system reach 73%. The stirring tank was turned on and kneaded for 2 hours to obtain a dough-like slurry.

[0029] The above-mentioned combined dispersant was dispersed with a small amount of solvent and then added to the dough-like slurry, and CNT and solvent N-methylpyrrolidone were added to the dough-like slurry to make the solid content of the slurry 50%. The high speed was turned on and the dispersion line speed was 30m / s to obtain the positive electrode slurry.

[0030] Example 2 The weight ratio of the slurry raw materials is sodium ferric sulfate: carbon black (SP): carbon nanotubes (CNTs): polyvinylidene fluoride (PVDF): combined dispersant = 94:1.5:0.2:4:0.3. The solvent is N-methylpyrrolidone; the combined dispersant is monophenyl phosphate and diphenyl phosphate, with a mass ratio of 7:3.

[0031] The preparation method comprises the following steps: The sodium ferric sulfate cathode material was baked in a 200°C oven for 24 hours, while SP and PVDF were baked at 60°C for 3 hours. After the materials cooled to room temperature, the sodium ferric sulfate, binder PVDF, and conductive agent SP were dry-mixed for 1 hour. The dry powder stacking order was 50% sodium ferric sulfate, SP, PVDF, and 50% sodium ferric sulfate, resulting in a uniform dry powder mixture.

[0032] The dry powder mixture was placed in a stirring tank, and solvent N-methylpyrrolidone was added to make the solid content of the dry powder mixture and solvent mixed system reach 73%. The stirring tank was turned on and kneaded for 2 hours to obtain a dough-like slurry.

[0033] The above-mentioned combined dispersant was dispersed with a small amount of solvent and then added to the dough-like slurry, and CNT and solvent N-methylpyrrolidone were added to the dough-like slurry to make the solid content of the slurry 50%. The high speed was turned on and the dispersion line speed was 30m / s to obtain the positive electrode slurry.

[0034] Example 3 The weight ratio of the slurry raw materials is sodium ferric sulfate: carbon black (SP): carbon nanotubes (CNT): polyvinylidene fluoride (PVDF): combined dispersant = 94:1.5:0.2:4:0.3, the solvent is N-methylpyrrolidone; the combined dispersant is monooctyl phosphate and bis (2-ethylhexyl) phosphate, and the mass ratio of the two is: 2:1.

[0035] The preparation method comprises the following steps: The sodium ferric sulfate cathode material was baked in a 200°C oven for 24 hours, while SP and PVDF were baked at 60°C for 3 hours. After the materials cooled to room temperature, the sodium ferric sulfate, binder PVDF, and conductive agent SP were dry-mixed for 1 hour. The dry powder stacking order was 50% sodium ferric sulfate, SP, PVDF, and 50% sodium ferric sulfate, resulting in a uniform dry powder mixture.

[0036] The dry powder mixture was placed in a stirring tank, and solvent N-methylpyrrolidone was added to make the solid content of the dry powder mixture and solvent mixed system reach 73%. The stirring tank was turned on and kneaded for 2 hours to obtain a dough-like slurry.

[0037] The above-mentioned combined dispersant was dispersed with a small amount of solvent and then added to the dough-like slurry, and CNT and solvent N-methylpyrrolidone were added to the dough-like slurry to make the solid content of the slurry 50%. The high speed was turned on and the dispersion line speed was 30m / s to obtain the positive electrode slurry.

[0038] Example 4 The weight ratio of the slurry raw materials is sodium ferric sulfate: carbon black (SP): carbon nanotubes (CNT): polyvinylidene fluoride (PVDF): combined dispersant = 94:1.5:0.2:4:0.3, the solvent is N-methylpyrrolidone; the combined dispersant is monooleyl phosphate and dioleyl phosphate, and the mass ratio of the two is: 1.5:1.

[0039] The preparation method comprises the following steps: The sodium ferric sulfate cathode material was baked in a 200°C oven for 24 hours, while SP and PVDF were baked at 60°C for 3 hours. After the materials cooled to room temperature, the sodium ferric sulfate, binder PVDF, and conductive agent SP were dry-mixed for 1 hour. The dry powder stacking order was 50% sodium ferric sulfate, SP, PVDF, and 50% sodium ferric sulfate, resulting in a uniform dry powder mixture.

[0040] The dry powder mixture was placed in a stirring tank, and solvent N-methylpyrrolidone was added to make the solid content of the dry powder mixture and solvent mixed system reach 73%. The stirring tank was turned on and kneaded for 2 hours to obtain a dough-like slurry.

[0041] The above-mentioned combined dispersant was dispersed with a small amount of solvent and then added to the dough-like slurry, and CNT and solvent N-methylpyrrolidone were added to the dough-like slurry to make the solid content of the slurry 50%. The high speed was turned on and the dispersion line speed was 30m / s to obtain the positive electrode slurry.

[0042] Comparative Example 1 This comparative example does not add a combined dispersant, and the other raw materials and steps are exactly the same as those in Example 4.

[0043] Comparative Example 2 In this comparative example, the combined dispersant in Example 4 was replaced with monooleyl phosphate, and the other raw materials and steps were exactly the same.

[0044] Comparative Example 3 In this comparative example, the combined dispersant in Example 4 was replaced with dioleyl phosphate, and the other raw materials and steps were exactly the same.

[0045] Viscosity determination: The positive electrode slurries obtained in the examples and comparative examples were allowed to stand for 4 hours each for viscosity testing. A rotary viscometer and a #4 rotor were used. The rotor was inserted into the slurry to be tested. The test speed was 10 rpm and the test time was 1 min. The slurry viscosity value was obtained. The test results are shown in the following table.

[0046] Table 1 Viscosity values ​​of various embodiments and comparative examples In each example, Example 1 used a C12-chain phosphate as a dispersant. The long dodecyl chain provided strong steric hindrance, resulting in high dispersion efficiency. The ester group exhibited stable adsorption, inhibiting particle reagglomeration, resulting in a viscosity increase of only 34% after 24 hours. Example 2 used a phenyl phosphate as a dispersant. The rigid structure of the benzene ring provided excellent dispersing properties, resulting in a gel-free slurry. However, the rigid structure of the benzene ring resulted in a thin adsorption layer, weak dispersibility, and a lack of effective positions. The viscosity increased by 68% within 24 hours. Example 3 used a C8 / C6 branched ester as a dispersant. The short chain and branched structure balanced adsorption and steric hindrance, resulting in a 43% increase in viscosity after 24 hours. Example 4 used an oleyl ester as a dispersant. The unsaturated double bond enhanced compatibility with the solvent, resulting in a tighter adsorption layer. The viscosity increased by only 28% after 24 hours. Comparative Example 1 did not include a dispersant, resulting in no dispersion protection for the particles. Van der Waals forces dominated agglomeration, and the viscosity reached 2.6 times the initial value after 24 hours. In Comparative Example 2, the highly polar phosphate groups of the monooleyl ester rapidly adsorbed onto the surface of the sodium ferric sulfate particles, reducing the viscosity to approximately 7,000 mPa·s in a short period. However, the high concentration of monoester (100%) exhibited poor compatibility with PVDF, potentially leading to phase separation and the appearance of "fish-eye" defects in the electrode after coating. This reliance solely on the electrostatic repulsion of the monoester, lacking the steric hindrance of the diester, led to a high risk of secondary particle agglomeration. The hydrophobic chains of the diester ester were difficult to effectively extend (the rigidity of the double bond limited the flexibility of the molecule), resulting in weak steric hindrance and poor dispersion. Under high shear rates (e.g., stirring), the diester molecules easily broke, causing a brief drop in viscosity followed by a rapid recovery. Furthermore, the extremely hydrophobic nature of the diester ester resulted in a low affinity for NMP, leading to poor slurry uniformity.

[0047] Based on the above embodiments and comparative examples, monoesters and diesters need to be compounded to balance electrostatic repulsion and steric hindrance, while increasing the solvent content to ensure that the dispersant is fully dissolved and micelle formation is reduced. A single phosphate dispersant is significantly inferior to the embodiments in both short-term dispersion effect and long-term stability, mainly due to molecular structure defects (double bond oxidation, crystallization tendency) and lack of synergistic effect. Therefore, for sodium iron sulfate battery slurry, an alkyl / branched ester and alkenyl ester compound system is preferably selected as a dispersant to obtain a gel-free, highly stable positive electrode slurry.

[0048] The above is a further detailed description of the present invention in conjunction with specific preferred embodiments, and the specific implementation of the present invention should not be considered to be limited to these descriptions. For those skilled in the art to which the present invention belongs, several simple deductions or substitutions can be made without departing from the concept of the present invention, and all of these should be considered to fall within the scope of protection of the present invention.

Claims

1. A gel-free high-stability sodium ion battery positive electrode slurry, characterized in that: The raw materials of the positive electrode slurry include: 90-96 parts of sodium ferric sulfate, 1-4 parts of conductive agent, 2-6 parts of binder, 0.1-0.5 parts of combined dispersant; The combined dispersant comprises: phosphoric acid monoester RO-PO(OH)2 and phosphoric acid diester (RO)2-PO(OH); wherein R is one or more of dodecyl, phenyl, octyl, oleyl, polyethylene glycol, and aminopropyl; The chemical formula of the sodium ferric sulfate is: Na x Fe 1-y (SO4) z , where 1.1≤x≤1.7, 0<y≤0.1, and x, y, and z are charge balanced.

2. The gel-free high-stability sodium ion battery positive electrode slurry according to claim 1, characterized in that: The mass ratio of the monoester of phosphate to the diester of phosphate in the combined dispersant is 1.2:1 to 7:3, specifically including any combination of monododecyl phosphate and didodecyl phosphate, monophenyl phosphate and diphenyl phosphate, monooctyl phosphate and bis(2-ethylhexyl) phosphate, and monooleyl phosphate and dioleyl phosphate.

3. The gel-free high-stability sodium ion battery positive electrode slurry according to claim 1, characterized in that: The conductive agent is one or more of carbon black, acetylene black, conductive graphite, and carbon nanotubes.

4. The gel-free high-stability sodium ion battery positive electrode slurry according to claim 1, characterized in that: The binder is one or more of polyvinylidene fluoride, polytetrafluoroethylene, and styrene-butadiene rubber.

5. The gel-free high-stability sodium ion battery positive electrode slurry according to claim 1, characterized in that: The positive electrode slurry further includes a solvent, which is one or more of N-methylpyrrolidone, N,N-dimethylformamide, and dimethyl sulfoxide.

6. The method for preparing the gel-free high-stability sodium ion battery positive electrode slurry according to any one of claims 1 to 5, characterized in that: The following steps are involved: S1. Drying sodium ferric sulfate, part of the conductive agent, and the binder and then mixing them to obtain a dry powder mixture; S2, mixing the dry powder mixture with a solvent and stirring to obtain a dough-like slurry; S3. Dispersing the combined dispersant with a solvent and adding it to the dough-like slurry, and adding the remaining conductive agent and solvent, mixing, stirring and dispersing to obtain a positive electrode slurry.

7. The method for preparing a gel-free high-stability sodium ion battery positive electrode slurry according to claim 6, characterized in that: The solid content of the dry powder mixture and the solvent mixture system in step S2 is 63-73%; the solid content of the positive electrode slurry in step S3 is 45-60%.

8. The method for preparing a gel-free high-stability sodium ion battery positive electrode slurry according to claim 6, characterized in that: The viscosity of the positive electrode slurry ranges from 5000 to 8000 CP.

9. A positive electrode sheet comprising the gel-free high-stability sodium ion battery positive electrode slurry according to any one of claims 1 to 5, characterized in that: The positive electrode slurry is coated by transfer coating and slit extrusion coating to form a positive electrode sheet for a sodium ion battery.

10. A sodium ion battery, characterized in that: The positive electrode sheet of the sodium ion battery is the positive electrode sheet according to claim 9.

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