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

Abstract

The application belongs to the field of sodium ion battery materials, and mainly relates to a gel-free high-stability positive electrode slurry as well as a preparation method and application thereof. The positive electrode slurry raw materials comprise: 90-96 parts of sodium iron 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: a monoester phosphate and a diester phosphate; wherein R in the ester group is one or more of dodecyl, phenyl, octyl, oleyl, polyethylene glycol group, and aminopropyl. The mixed system of the monoester phosphate and the diester phosphate is used as the dispersant, that is, the mixed system has high affinity with iron ions, locks the active metal ions on the particle surface, blocks the side reaction with the solvent, the short chain of the monoester quickly anchors the particle surface, the long chain of the diester extends into the solvent to form steric hindrance, and the double mechanism inhibits agglomeration, thereby improving the dispersibility and stability of the slurry.

Description

Technical Field

[0001] This invention belongs to the field of battery materials technology, and mainly relates to a gel-free, highly stable cathode slurry, its preparation method, and its application. Background Technology

[0002] Sodium iron sulfate (NaFeSO4) has attracted much attention as a cathode material for sodium-ion batteries due to its low cost and high safety, but its electrode slurry preparation faces severe challenges: the Fe²⁺ on the particle surface... + / Fe³ + It has high ionic activity and readily absorbs water or occupies the binding sites of NMP and PVDF, leading to slurry agglomeration and a surge in viscosity; at the same time, the material's polar surface has poor compatibility with organic solvents.

[0003] To address these issues, carboxylic acid reagents are currently the primary dispersants. For example, in high-flowability sodium ferric sulfate slurry for sodium-ion batteries, the flowability modifier used is the dicarboxylic acid compound HOOC-R-COOH. This dispersant disperses the material by forming M-OOC- bonds through chemical adsorption of the carboxylic acid groups onto the metal oxides on the surface of the active material. However, this method is ineffective at dispersing hydrophobic materials in the slurry, such as conductive agents like carbon black, as it struggles to disrupt π-π packing. Furthermore, it exhibits poor thermal stability, undergoing decarboxylation at 120℃. Its acidic nature may also corrode the current collector, increasing the risk of self-discharge. Additionally, its strong adsorption competes with PVDF for binding sites, further complicating dispersion. In short, traditional dispersants struggle to effectively suppress interparticle van der Waals forces and electrostatic attraction, increasing the likelihood of slurry gelation and resulting in uneven porosity in the coated film, directly impacting battery rate performance and cycle life. Summary of the Invention

[0004] To address the gelation problem of sodium ferric sulfate homogenates and the high cost and poor adhesion resulting from low-solids homogenate coatings, this invention proposes adding a mixture of monophosphate / diester phosphate to disperse the slurry and improve the coating's solids content. The synergistic effect of strong adsorption and ion complexation, along with precise control of surface charge, enhances the slurry's dispersion effect and reduces costs.

[0005] In an embodiment of the present invention, the present invention provides a gel-free, highly stable sodium-ion battery cathode slurry, wherein the raw materials of the cathode slurry include:

[0006] The sodium ferric sulfate comprises 90-96 parts of sodium ferric sulfate, 1-4 parts of conductive agent, 2-6 parts of binder, and 0.1-0.5 parts of combined dispersant;

[0007] The combined dispersant, by percentage content, comprises: 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.

[0008] The chemical formula is: Na x Fe 1-y (SO4) z , where 1.1≤x≤1.7, 0<y≤0.1, and x, y, z are in charge balance.

[0009] In a specific embodiment, the mass ratio of mono- and diester phosphates in the combined dispersant is 1.2:1 to 7:3, specifically including any combination of mono- and di-dodecyl phosphate, mono- and di-phenyl phosphate, mono- and di-octyl phosphate, mono- and di-octyl phosphate, and mono- and di-oleyl phosphate.

[0010] In a specific embodiment, the conductive agent is one or more of carbon black, conductive graphite, and carbon nanotubes.

[0011] In a specific embodiment, the adhesive is one or more of polyvinylidene fluoride, polytetrafluoroethylene, and styrene-butadiene rubber.

[0012] In a specific embodiment, the positive electrode slurry further includes a solvent, which is one or more of N-methylpyrrolidone, N,N-dimethylformamide, and dimethyl sulfoxide.

[0013] Based on the same inventive concept, the present invention also provides a method for preparing the above-mentioned gel-free, highly stable sodium-ion battery cathode slurry, comprising the following steps:

[0014] S1. Dry sodium ferric sulfate, a portion of conductive agent, and binder are dried and then mixed to obtain a dry powder mixture.

[0015] S2. Mix the dry powder mixture with a solvent and stir to obtain a dough-like slurry;

[0016] S3. The combined dispersant is dispersed in a solvent and then added to the dough-like slurry. The remaining conductive agent and solvent are added and mixed and dispersed to obtain the positive electrode slurry.

[0017] In a specific embodiment, a solvent is added in step S2, and the solid content of the dry powder mixture and solvent mixture system is 63-73%; a solvent is added in step S3, and the solid content of the positive electrode slurry is 45-60%.

[0018] In a specific embodiment, the viscosity range of the positive electrode slurry is 5000~8000 cP.

[0019] Based on the same inventive concept, this invention 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 to form the sodium-ion battery positive electrode sheet by transfer coating and slot extrusion coating.

[0020] The present invention also provides a sodium-ion battery, wherein the positive electrode of the sodium-ion battery is the aforementioned positive electrode.

[0021] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0022] This invention proposes adding a mixture of monophosphate / diester phosphate as a dispersant in the positive electrode slurry of the sodium ferric sulfate system, wherein the phosphate groups in the phosphate ester molecules are associated with Fe³⁺. + With high affinity, it can "lock" active metal ions on the particle surface through complexation, blocking their side reaction pathways with the solvent. Simultaneously, the mixed system of monoesters and diesters can form a gradient adsorption layer; the short chains of monoesters rapidly anchor to the particle surface, while the long chains of diesters extend into the solvent, creating steric hindrance—a dual mechanism to inhibit aggregation. Furthermore, the ionization properties of phosphate esters can regulate the zeta potential of the particle surface, enhancing the electrostatic repulsion effect. Compared to dicarboxylic acid dispersants, phosphate esters exhibit superior stability over a wide pH range, avoiding dispersion failure caused by pH fluctuations in carboxylic acids, and significantly improving the dispersibility and stability of the slurry. Detailed Implementation

[0023] To facilitate understanding of the present invention, the present invention will be described more fully and in detail below with reference to the accompanying drawings and preferred embodiments, but the scope of protection of the present invention is not limited to the following specific embodiments.

[0024] Unless otherwise defined, all technical terms used herein have the same meaning as commonly understood by those skilled in the art. The technical terms used herein are for the purpose of describing particular embodiments only and are not intended to limit the scope of the invention.

[0025] Unless otherwise specified, all raw materials, reagents, instruments and equipment used in this invention can be purchased from the market or prepared by existing methods.

[0026] This invention provides a gel-free, highly stable sodium-ion battery cathode slurry. The raw materials of the cathode 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, by percentage, includes: monophosphate ester RO-PO(OH)2 and diephosphate ester (RO)2-PO(OH); wherein R is one or more of dodecyl, phenyl, octyl, oleylene, 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, z satisfy charge balance.

[0027] Furthermore, the mass ratio of mono- and die-ester phosphates in the combined dispersant is 1.2:1 to 7:3, specifically including the following combinations: mono- and di-dodecyl phosphate, mono- and di-phenyl phosphate, mono- and di-octyl phosphate, mono- and di-octyl phosphate, and mono- and di-oleyl 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.

[0028] To obtain a gel-free, highly stable sodium-ion battery cathode slurry, the preparation method provided by this invention includes the following steps:

[0029] S1. Sodium ferric sulfate, a portion of the conductive agent, and the binder are dried and then mixed to obtain a dry powder mixture. The drying temperature in this process is determined based on their specific characteristics. For example, the evaporation temperature of the water of crystallization of sodium ferric sulfate is greater than or equal to 175℃. Therefore, the sodium ferric sulfate positive electrode material is baked at 200℃ for 24 hours, while the conductive agent and binder are dried at 60℃ for 3 hours. During the dry powder mixing process, the stacking order of the dry powder is 50% sodium ferric sulfate, conductive agent, binder, and then 50% sodium ferric sulfate, and dry-mixed for 1 hour or more to obtain the dry powder mixture.

[0030] S2. The dry powder mixture is mixed with a solvent and stirred to obtain a dough-like slurry. The amount of solvent added during this process is such that the solid content of the dry powder mixture and solvent mixture system is 63-73%. The mixture is then placed in a mixing tank and kneaded for more than 2 hours to obtain a dough-like slurry.

[0031] S3. The combined dispersant is dispersed in a solvent and then added to the dough-like slurry. The remaining conductive agent and solvent are added and mixed and dispersed to obtain the positive electrode slurry. During this process, the amount of solvent added ensures the solid content of the slurry is between 45% and 60%. A high-speed rotation is used, resulting in a dispersion linear velocity of 20-30 m / s. The viscosity of the positive electrode slurry obtained after stirring is in the range of 5000-8000 CP.

[0032] Based on the same inventive concept, this invention also provides a positive electrode sheet comprising the above-mentioned gel-free, high-stability sodium-ion battery positive electrode slurry. During the preparation process, the positive electrode slurry is coated using transfer coating and slot extrusion coating methods to form the sodium-ion battery positive electrode sheet. This invention also provides a sodium-ion battery, wherein the positive electrode sheet of the sodium-ion battery is the above-mentioned positive electrode sheet.

[0033] The following is a further explanation using specific embodiments.

[0034] Example 1

[0035] This embodiment provides a gel-free, high-stability sodium-ion battery cathode slurry raw material and its preparation method, including the following steps:

[0036] The slurry raw material has a weight ratio of sodium ferric sulfate: carbon black (SP): carbon nanotubes (CNT): polyvinylidene fluoride (PVDF): combined dispersant = 94:1.5:0.2:4:0.3, and the solvent is N-methylpyrrolidone; the combined dispersant is a mixture of mono-dodecyl phosphate and di-dodecyl phosphate, with a mass ratio of 7:3.

[0037] The preparation method includes the following steps:

[0038] The sodium ferric sulfate cathode material was baked in an oven at 200℃ for 24 hours, and SP and PVDF were baked at 60℃ for 3 hours. After the materials cooled to room temperature, sodium ferric sulfate, binder PVDF, and conductive agent SP were dry-mixed for 1 hour. The dry powder was stacked in the following order: 50% sodium ferric sulfate, SP, PVDF, and 50% sodium ferric sulfate, to obtain a uniformly mixed dry powder mixture.

[0039] Add the dry powder mixture to a mixing tank, add the solvent N-methylpyrrolidone, so that the solid content of the dry powder mixture and solvent mixture is 73%. Start the mixing tank to knead for 2 hours to obtain a dough-like slurry.

[0040] The above-mentioned combined dispersant was dispersed in a small amount of solvent and then added to the dough-like slurry. CNT and the solvent N-methylpyrrolidone were also added to the dough-like slurry to make the solid content of the slurry 50%. The slurry was dispersed at a high speed and a dispersion linear velocity of 30 m / s to obtain the positive electrode slurry.

[0041] Example 2

[0042] The slurry raw material composition by weight 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 monophenyl phosphate and diphenyl phosphate, with a mass ratio of 7:3.

[0043] The preparation method includes the following steps:

[0044] The sodium ferric sulfate cathode material was baked in an oven at 200℃ for 24 hours, and SP and PVDF were baked at 60℃ for 3 hours. After the materials cooled to room temperature, sodium ferric sulfate, binder PVDF, and conductive agent SP were dry-mixed for 1 hour. The dry powder was stacked in the following order: 50% sodium ferric sulfate, SP, PVDF, and 50% sodium ferric sulfate, to obtain a uniformly mixed dry powder mixture.

[0045] Add the solvent N-methylpyrrolidone to the dry powder mixture in the mixing tank, so that the solid content of the dry powder mixture and solvent mixture is 73%. Start the mixing tank to knead for 2 hours to obtain a dough-like slurry.

[0046] The above-mentioned combined dispersant was dispersed in a small amount of solvent and then added to the dough-like slurry. CNT and the solvent N-methylpyrrolidone were also added to the dough-like slurry to make the solid content of the slurry 50%. The slurry was dispersed at a high speed and a dispersion linear velocity of 30 m / s to obtain the positive electrode slurry.

[0047] Example 3

[0048] The weight ratio of the raw materials for the slurry is sodium ferric sulfate: carbon black (SP): carbon nanotubes (CNT): polyvinylidene fluoride (PVDF): combined dispersant = 94:1.5:0.2:4:0.3, and 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.

[0049] The preparation method includes the following steps:

[0050] The sodium ferric sulfate cathode material was baked in an oven at 200℃ for 24 hours, and SP and PVDF were baked at 60℃ for 3 hours. After the materials cooled to room temperature, sodium ferric sulfate, binder PVDF, and conductive agent SP were dry-mixed for 1 hour. The dry powder was stacked in the following order: 50% sodium ferric sulfate, SP, PVDF, and 50% sodium ferric sulfate, to obtain a uniformly mixed dry powder mixture.

[0051] Add the solvent N-methylpyrrolidone to the dry powder mixture in the mixing tank, so that the solid content of the dry powder mixture and solvent mixture is 73%. Start the mixing tank to knead for 2 hours to obtain a dough-like slurry.

[0052] The above-mentioned combined dispersant was dispersed in a small amount of solvent and then added to the dough-like slurry. CNT and the solvent N-methylpyrrolidone were also added to the dough-like slurry to make the solid content of the slurry 50%. The slurry was dispersed at a high speed and a dispersion linear velocity of 30 m / s to obtain the positive electrode slurry.

[0053] Example 4

[0054] 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, and the solvent is N-methylpyrrolidone; the combined dispersant is monooleenyl phosphate and dioleenyl phosphate, and the mass ratio of the two is 1.5:1.

[0055] The preparation method includes the following steps:

[0056] The sodium ferric sulfate cathode material was baked in an oven at 200℃ for 24 hours, and SP and PVDF were baked at 60℃ for 3 hours. After the materials cooled to room temperature, sodium ferric sulfate, binder PVDF, and conductive agent SP were dry-mixed for 1 hour. The dry powder was stacked in the following order: 50% sodium ferric sulfate, SP, PVDF, and 50% sodium ferric sulfate, to obtain a uniformly mixed dry powder mixture.

[0057] Add the solvent N-methylpyrrolidone to the dry powder mixture in the mixing tank, so that the solid content of the dry powder mixture and solvent mixture is 73%. Start the mixing tank to knead for 2 hours to obtain a dough-like slurry.

[0058] The above-mentioned combined dispersant was dispersed in a small amount of solvent and then added to the dough-like slurry. CNT and the solvent N-methylpyrrolidone were also added to the dough-like slurry to make the solid content of the slurry 50%. The slurry was dispersed at a high speed and a dispersion linear velocity of 30 m / s to obtain the positive electrode slurry.

[0059] Comparative Example 1

[0060] This comparative example does not use any combined dispersant, but the raw materials and steps are exactly the same as in Example 4.

[0061] Comparative Example 2

[0062] In this comparative example, the combined dispersant in Example 4 was replaced with monoolein phosphate, while all other raw materials and steps were exactly the same.

[0063] Comparative Example 3

[0064] In this comparative example, the combined dispersant in Example 4 was replaced with dioleyl phosphate, while all other raw materials and steps were exactly the same.

[0065] Viscosity measurement:

[0066] The positive electrode slurries obtained in the examples and comparative examples were subjected to viscosity tests every 4 hours. A rotational viscositometer 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 viscosity values ​​of the slurry were obtained, and the test results are shown in the table below.

[0067] Table 1 Viscosity values ​​for each embodiment and comparative example

[0068]

[0069] In each embodiment, Example 1 used a C12 chain phosphate ester as a dispersant. The long dodecyl chain provided strong steric hindrance, resulting in high dispersion efficiency. The ester group adsorption was stable, inhibiting particle re-agglomeration, and the viscosity increased by only 34% after 24 hours. Example 2 used a phenyl phosphate ester as a dispersant. The rigid structure of the benzene ring had good dispersing effect, forming a gel-free slurry. However, the rigid structure of the benzene ring resulted in a thin adsorption layer, weak dispersion ability, and a lack of effective sites, leading to a 68% increase in viscosity within 24 hours. In Example 3, a C8 / C6 branched ester was used as a dispersant. The short chain and branched structure balanced adsorption and steric hindrance, resulting in a 43% increase in viscosity after 24 hours. In Example 4, an oleyl ester was used as a dispersant. The unsaturated double bond enhanced the compatibility with the solvent, resulting in a tighter adsorption layer, and the viscosity increase was only 28% after 24 hours. Comparative Example 1 did not contain a dispersant, and the particles had no dispersion protection. 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 monoolein ester can be rapidly adsorbed onto the surface of sodium ferric sulfate particles, reducing the viscosity to around 7,000 mPa·s in a short period. However, the high concentration of monoolein (100%) has poor compatibility with PVDF, which may lead to phase separation. After coating, the electrode exhibits "fish-eye" defects. Relying solely on the electrostatic repulsion of the monoolein, and lacking the steric hindrance synergy of the diester, the risk of secondary particle agglomeration is high. The hydrophobic chain of the diesterin ester is difficult to extend effectively (the rigidity of the double bond restricts the flexibility of the molecule), resulting in weak steric hindrance and poor dispersion. Under high shear rates (such as stirring), the diester molecules are easily broken, and the viscosity rapidly recovers after a brief decrease. At the same time, the diesterin ester is extremely hydrophobic and has low affinity for NMP, leading to poor slurry uniformity.

[0070] Based on the above examples and comparative examples, monoesters and diesters need to be used in combination to balance electrostatic repulsion and steric hindrance, while increasing the solvent content to ensure that the dispersant is fully dissolved and reduce micelle formation. Single phosphate ester dispersants are significantly inferior to the examples in terms of 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, alkyl / branched ester and alkenyl ester compound system is preferred as dispersant to obtain gel-free high-stability cathode slurry.

[0071] The above description, in conjunction with specific preferred embodiments, provides a further detailed explanation of the present invention. However, it should not be construed that the specific implementation of the present invention is limited to these descriptions. For those skilled in the art, various simple deductions or substitutions can be made without departing from the concept of the present invention, and all such modifications and substitutions should be considered within the scope of protection of the present invention.

Claims

1. A gel-free, high-stability sodium-ion battery cathode slurry, characterized in that, The raw materials for the positive electrode slurry include: Sodium ferric sulfate 90-96 parts, conductive agent 1-4 parts, binder 2-6 parts, combined dispersant 0.1-0.5 parts; The combined dispersant comprises: 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 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, z are in charge balance.

2. The gel-free, high-stability sodium-ion battery cathode slurry according to claim 1, characterized in that, The mass ratio of mono- and die-grade phosphates in the combined dispersant is 1.2:1 to 7:3, specifically including any combination of mono- and di-dodecyl phosphates, mono- and di-phenyl phosphates, mono- and di-octyl phosphates, mono- and di-octyl phosphates, and mono- and di-oleyl phosphates.

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

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

5. The gel-free, high-stability sodium-ion battery cathode slurry according to claim 1, characterized in that, The positive electrode slurry also 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, highly stable sodium-ion battery cathode slurry according to any one of claims 1 to 5, characterized in that, Includes the following steps: S1. Dry sodium ferric sulfate, a portion of conductive agent, and binder are dried and then mixed to obtain a dry powder mixture. S2. Mix the dry powder mixture with a solvent and stir to obtain a dough-like slurry; S3. The combined dispersant is dispersed in a solvent and then added to the dough-like slurry. The remaining conductive agent and solvent are added and mixed and dispersed to obtain the positive electrode slurry.

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

8. The method for preparing gel-free, high-stability sodium-ion battery cathode slurry according to claim 6, characterized in that, The viscosity range of the positive electrode slurry is 5000~8000 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 using transfer coating and slot extrusion coating methods to form the positive electrode sheet of a sodium-ion battery.

10. A sodium-ion battery, characterized in that, The positive electrode of the sodium-ion battery is the positive electrode as described in claim 9.

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