A sulfate-based sodium-ion positive electrode material and a preparation method thereof

Abstract

This invention belongs to the field of sodium battery technology, specifically relating to a sulfate-based sodium-ion cathode material and its preparation method. The preparation method includes: mixing Na₂Mn(SO₄)₂ with conductive polyaniline and then vacuum dry ball milling to obtain a mixed powder; adding the mixed powder to a hydrothermal reactor lined with alumina-corundum ceramic containing N-methylpyrrolidone solvent, adjusting the pH to 3-4 with citric acid, and reacting hydrothermally at 250-300°C; washing the reaction product with sodium carbonate solution and then vacuum drying to obtain the final product. This invention uses conductive polyaniline as a pre-finished product, eliminating the need for oxidants. Through a hydrothermal reaction, the sulfate ions released from Na₂Mn(SO₄)₂ undergo reversible anion doping with the polyaniline, anchoring the sulfate ions at the molecular level, effectively suppressing SO₂ gas production during electrochemical cycling, and avoiding battery gas expansion problems. This invention features a simple, safe, and environmentally friendly process, and the resulting material exhibits excellent cycle stability.

Description

Technical Field

[0001] This invention belongs to the field of sodium battery technology, specifically relating to a sulfate-based sodium ion cathode material and its preparation method. Background Technology

[0002] Sodium-ion batteries are a novel electrochemical energy storage technology that offers advantages over lithium-ion batteries, including higher safety, lower cost, and access to more abundant sodium resources. In cathode material systems, polyanionic sulfate cathode materials, due to the higher electronegativity of sulfate groups compared to other polyanionic groups, can provide a high operating voltage approaching 3.8V. Furthermore, their simple preparation process and low cost make them one of the most promising cathode material systems for sodium-ion batteries. In addition, sulfate materials exhibit low volume change during cycling, demonstrating good cycle stability.

[0003] However, the practical application of sulfate-based cathode materials still faces challenges. First, the intrinsic electronic conductivity of sulfate materials is extremely low, which limits their rate performance and capacity. Second, sulfate materials are prone to decomposition during electrochemical cycling. Sulfate ions generate SO2 gas through electrochemical reactions during charge and discharge, leading to battery swelling, performance degradation, and even safety hazards, seriously affecting the battery's cycle life.

[0004] Chinese patent CN119560511A discloses a high-conductivity sodium-ion battery cathode material / organic conductive polymer composite material and its preparation method. First, the organic conductive polymer monomer is polymerized in an acidic aqueous solution using an oxidizing agent. Then, a sodium source, an iron source, and a sulfur source are added to the solution to form a precursor slurry. After drying, the slurry is sintered at 300–450°C to obtain Na… 2+x Fe 2-x (SO4)3 / organic conductive polymer composite cathode material. This method improves the electronic conductivity of the material to some extent. However, this technical solution still has the following shortcomings: (1) An oxidant must be added to initiate polymerization. Monomer polymerization is induced by oxidants (hydrogen peroxide, hydrazine hydrate, etc.). Residual oxidants will introduce additional impurities, exacerbate side reactions, and have an adverse effect on the electrochemical performance of the cathode material.

[0005] (2) The conductive polymer is only physically coated with the matrix and lacks chemical bonding. The polymer exists only as a coating layer on the surface of the active material particles. The interfacial bonding strength is limited. During long-term cycling, the coating layer is easy to peel off and fall off, and the stabilization of sulfate cannot be achieved.

[0006] (3) The problem of SO2 generation during the electrochemical cycle was not solved. The background technology only mentions that SO2 decomposition should be avoided during the high-temperature preparation of sulfate materials, so the sintering temperature is controlled below 450°C to passively avoid it, which is a physical protection at the preparation process level. However, the problem of SO2 generation driven by electrochemical reaction during the charge-discharge cycle of the battery is not addressed, and the physical coating modification method cannot suppress it.

[0007] Therefore, how to maintain the advantages of high voltage and low volume change of sulfate materials while avoiding SO2 gas generation during electrochemical cycling is a technical problem that urgently needs to be solved in this field. Summary of the Invention

[0008] To address the shortcomings of existing technologies, the present invention aims to provide a sulfate-based sodium ion cathode material that can effectively suppress SO2 generation during electrochemical cycling, prevent battery gas expansion, and significantly improve cycle stability. The present invention also provides a preparation method that does not require the addition of oxidants or initiators, is simple in process, safe and environmentally friendly, low in cost, and suitable for large-scale production.

[0009] The objective of this invention is achieved through the following technical solution: The preparation method of the sulfate-based sodium ion cathode material includes the following steps: (1) Na2Mn(SO4)2 and conductive polyaniline were mixed and then dry ball milled under vacuum to obtain a mixed powder; (2) Add the mixed powder into an alumina corundum ceramic-lined hydrothermal reactor containing N-methylpyrrolidone solvent, adjust the pH value to 3-4, and then place it in a muffle furnace for hydrothermal reaction at 250-300℃. Maintaining a pH value of 3-4 provides a suitable acidic environment that helps polyaniline retain its conductive doped structure and inhibits Mn production. 2+ Hydrolysis precipitation; at the same time, regulating the redox potential to ensure the smooth occurrence of the redox reaction and providing anions to achieve charge balance.

[0010] The temperature should be controlled between 250 and 300℃. Too low a temperature results in a slow reaction rate and incomplete conversion; too high a temperature causes dedoping and degradation of polyaniline, while simultaneously promoting the de-doping of Mn. 2+ Oxidation produces impurities; a suitable temperature is needed to balance the reaction rate, maintain the conductive structure of polyaniline, and suppress side reactions.

[0011] (3) The powder obtained after hydrothermal reaction is washed with sodium carbonate solution and then vacuum dried to obtain sulfate sodium ion cathode material.

[0012] In step (1), the mass ratio of Na2Mn(SO4)2 to conductive polyaniline is 40:(2-3). This mass ratio allows polyaniline to form a uniform thin conductive coating; too little polyaniline makes it difficult to construct a complete conductive network, and manganese dissolution is aggravated; too much polyaniline results in an excessively thick coating layer, which hinders ion transport and reduces the specific capacity of the composite material.

[0013] In step (1), the weight-average molecular weight of conductive polyaniline is 60,000 to 70,000. If the molecular weight is too low, the conductivity is poor and the coating is easy to fall off; if the molecular weight is too high, the molecules will entangle and agglomerate, hindering ion diffusion; a moderate molecular weight is conducive to forming a uniform and stable conductive coating layer.

[0014] In step (1), a planetary ball mill is used for dry ball milling. The grinding balls are agate balls, the ball-to-material mass ratio is (10-12):1, the ball milling speed is 150-300 rpm, and the ball milling time is 3-5 h.

[0015] In step (2), the hydrothermal reaction takes 6 to 10 hours.

[0016] In step (2), the heating rate of the hydrothermal reaction is 1-3℃ / min. Excessive heating causes particle agglomeration, localized thermal degradation of polyaniline, and uneven coating; excessively slow heating easily leads to grain growth, prolonged oxidative degradation of polyaniline, and low efficiency; a gentle heating rate of 1-3℃ / min ensures a uniform temperature field, achieves uniform coating, controllably regulates crystal growth, and protects the conductive structure of polyaniline.

[0017] In step (2), the amount of N-methylpyrrolidone added accounts for 60-70% of the volume of the hydrothermal reactor lined with alumina corundum ceramic.

[0018] In step (2), citric acid is used to adjust the pH value. Citric acid can dissociate H+... + Under acidic conditions, citrate ions can also complex Mn. 2+ It regulates grain growth, inhibits aggregation, and has good pH buffering properties.

[0019] In step (3), the concentration of the sodium carbonate solution is 1–3 mol / L. If the concentration is too low, the impurity removal will be incomplete; if the concentration is too high, the alkalinity will be too strong, causing manganese ions to hydrolyze and form precipitates, and polyaniline to lose its doping. A concentration of 1–3 mol / L can efficiently remove impurities while ensuring that the structure of the active material and the conductive structure of polyaniline are not damaged.

[0020] In step (3), the vacuum drying temperature is 90-120℃ and the drying time is 10-12h.

[0021] The sulfate-based sodium ion cathode material of the present invention is prepared by the above-described preparation method.

[0022] Compared with the prior art, the beneficial effects of the present invention are as follows: (1) This invention uses conductive polyaniline as a finished product. Through hydrothermal reaction, sulfate ions released from Na2Mn(SO4)2 are reversibly anion-doped with it, anchoring sulfate ions at the molecular level. This effectively suppresses SO2 gas generation during electrochemical cycling, avoids battery gas swelling, and significantly improves cycle stability. The cathode material prepared using the method of this invention retains more than 95% of its capacity after 50 cycles.

[0023] (2) The present invention directly adds conductive polyaniline finished product without the need for oxidant or initiator, avoiding the introduction of impurities. The process is simple, safe and environmentally friendly, and low in cost, making it suitable for large-scale production.

[0024] (3) The present invention achieves a strong bond between polyaniline and sulfate matrix through chemical bonding, ensuring structural stability during long-term cycling. Detailed Implementation

[0025] The present invention will be further described below with reference to the embodiments, but these embodiments do not limit the implementation of the present invention.

[0026] Unless otherwise specified, the raw materials used in the examples and comparative examples are all commercially available materials, and the process methods used in the examples and comparative examples are all conventional methods in the art.

[0027] The conductive polyaniline used in the examples has a weight-average molecular weight of approximately 65,000 and is a commercially available product from Merck.

[0028] Example 1 The preparation method of the sulfate-based sodium ion cathode material comprises the following steps: (1) Na2Mn(SO4)2 and conductive polyaniline were mixed at a mass ratio of 40:2. After mixing, the mixture was placed in a planetary ball mill with agate balls as grinding balls and a ball-to-material mass ratio of 10:1. Dry ball milling was carried out at a ball mill speed of 150 rpm and a ball milling time of 3 h to obtain mixed powder. (2) The above mixed powder is added into an alumina corundum ceramic-lined hydrothermal reactor containing N-methylpyrrolidone solvent. The amount of N-methylpyrrolidone added accounts for 60% of the reactor volume. The pH of the mixed solvent is adjusted to 3 with citric acid. The mixture is then placed in a muffle furnace for hydrothermal reaction. The hydrothermal reaction requires heating at 250°C for 6 hours with a heating rate of 1°C / min. (3) The powder obtained after the hydrothermal reaction is washed with sodium carbonate solution with a concentration of 1 mol / L.

[0029] (4) The cleaned material is vacuum dried at a temperature of 90°C for 10 hours.

[0030] Example 2 The preparation method of the sulfate-based sodium ion cathode material comprises the following steps: (1) Na2Mn(SO4)2 and conductive polyaniline were mixed at a mass ratio of 40:3. After mixing, the mixture was placed in a planetary ball mill with agate balls as grinding balls and a ball-to-material mass ratio of 11:1. Dry ball milling was carried out at a ball mill speed of 150 rpm and a ball milling time of 4 h to obtain mixed powder. (2) The above mixed powder is added into an alumina corundum ceramic-lined hydrothermal reactor containing N-methylpyrrolidone solvent. The amount of N-methylpyrrolidone added accounts for 70% of the reactor volume. The pH of the mixed solvent is adjusted to 3.5 with citric acid. The mixture is then placed in a muffle furnace for hydrothermal reaction. The hydrothermal reaction requires heating at 280℃ for 7 hours with a heating rate of 2℃ / min. (3) The powder obtained after the hydrothermal reaction is washed with sodium carbonate solution with a concentration of 2 mol / L.

[0031] (4) The cleaned material is vacuum dried at a temperature of 100°C for 11 hours.

[0032] Example 3 The preparation method of the sulfate-based sodium ion cathode material comprises the following steps: (1) Na2Mn(SO4)2 and conductive polyaniline were mixed at a mass ratio of 40:2. After mixing, the mixture was placed in a planetary ball mill with agate balls as grinding balls and a ball-to-material mass ratio of 12:1. Dry ball milling was carried out at a ball mill speed of 200 rpm and a ball milling time of 5 h to obtain mixed powder. (2) The above mixed powder is added into an alumina corundum ceramic-lined hydrothermal reactor containing N-methylpyrrolidone solvent. The amount of N-methylpyrrolidone added accounts for 65% of the reactor volume. The pH of the mixed solvent is adjusted to 4 with citric acid. The mixture is then placed in a muffle furnace for hydrothermal reaction. The hydrothermal reaction requires heating at 300℃ for 8 hours with a heating rate of 3℃ / min. (3) The powder obtained after the hydrothermal reaction is washed with sodium carbonate solution with a concentration of 2 mol / L.

[0033] (4) The cleaned material is vacuum dried at a temperature of 100°C for 11 hours.

[0034] Example 4 The preparation method of the sulfate-based sodium ion cathode material comprises the following steps: (1) Na2Mn(SO4)2 and conductive polyaniline were mixed at a mass ratio of 40:3. After mixing, the mixture was placed in a planetary ball mill with agate balls as grinding balls and a ball-to-material mass ratio of 11:1. Dry ball milling was carried out at a ball mill speed of 250 rpm and a ball milling time of 4 h to obtain mixed powder. (2) The above mixed powder is added into an alumina corundum ceramic-lined hydrothermal reactor containing N-methylpyrrolidone solvent. The amount of N-methylpyrrolidone added accounts for 60% of the reactor volume. The pH of the mixed solvent is adjusted to 3.5 with citric acid. The mixture is then placed in a muffle furnace for hydrothermal reaction. The hydrothermal reaction requires heating at 280℃ for 10 hours with a heating rate of 2℃ / min. (3) The powder obtained after the hydrothermal reaction is washed with sodium carbonate solution with a concentration of 2 mol / L.

[0035] (4) The cleaned material is vacuum dried at a temperature of 110°C for 12 hours.

[0036] Example 5 The preparation method of the sulfate-based sodium ion cathode material comprises the following steps: (1) Na2Mn(SO4)2 and conductive polyaniline were mixed at a mass ratio of 40:2. After mixing, the mixture was placed in a planetary ball mill and agate balls were used as grinding balls. The ball-to-material mass ratio was 10:1. Dry ball milling was carried out at a ball mill speed of 300 rpm and a ball milling time of 3 h to obtain mixed powder. (2) The above mixed powder is added into an alumina corundum ceramic-lined hydrothermal reactor containing N-methylpyrrolidone solvent. The amount of N-methylpyrrolidone added accounts for 70% of the reactor volume. The pH of the mixed solvent is adjusted to 3 with citric acid. The mixture is then placed in a muffle furnace for hydrothermal reaction. The hydrothermal reaction requires heating at 250°C for 10 hours with a heating rate of 1°C / min. (3) The powder obtained after the hydrothermal reaction is washed with sodium carbonate solution with a concentration of 2 mol / L.

[0037] (4) The cleaned material is vacuum dried at a temperature of 100°C for 10 hours.

[0038] Comparative Example 1 The only difference between this comparative example and Example 1 is that conductive polyaniline is not added in step (1).

[0039] Comparative Example 2 The only difference between this comparative example and Example 2 is that, in step (2), instead of the hydrothermal method, a direct vacuum heating method is adopted, and the muffle furnace is heated at 280°C for 7 hours after evacuation.

[0040] Comparative Example 3 The only difference between this comparative example and Example 3 is that, in step (2), the pH is set to 2. Comparative Example 4 The only difference between this comparative example and Example 3 is that the pH is set to 5 in step (2).

[0041] Comparative Example 5 The only difference between this comparative example and Example 4 is that the hydrothermal reaction in step (2) is carried out at 230°C.

[0042] Comparative Example 6 The only difference between this comparative example and Example 4 is that the hydrothermal reaction in step (2) is carried out at 320°C.

[0043] Comparative Example 7 The only difference between this comparative example and Example 5 is that in step (1), the ratio of Na2Mn(SO4)2 to conductive polyaniline is 40:1.

[0044] Comparative Example 8 The only difference between this comparative example and Example 5 is that in step (1), the ratio of Na2Mn(SO4)2 to conductive polyaniline is 40:4.

[0045] Comparative Example 9 The only difference between this comparative example and Example 5 is that in step (1), conductive polyaniline with a weight average molecular weight of approximately 20,000, produced by the same manufacturer (Merck), was used.

[0046] Comparative Example 10 The only difference between this comparative example and Example 5 is that in step (1), conductive polyaniline with a weight-average molecular weight of approximately 100,000, produced by the same manufacturer (Merck), was used.

[0047] Full cells were assembled using the materials prepared in the examples and comparative examples. The specific battery assembly process is as follows: Hard carbon was used as the negative electrode, and the prepared material was used as the positive electrode. Positive electrode sheets of 96mm × 81mm and negative electrode sheets of 98mm × 83mm were fabricated. 1M NaPF6 was used as the electrolyte, and the organic solvent had a volume ratio of EC:DMC:DEC (ethylene carbonate: dimethyl carbonate: diethyl carbonate) = 1:1:1. A ceramic separator was used to assemble the soft-pack sodium-ion battery, with an NP ratio of 1.1:1. Charge-discharge and cycle performance tests were conducted at 1C. The initial discharge capacity and capacity retention after 50 cycles are shown in Table 1.

[0048] Table 1. Test results of the first discharge capacity and cycle life of the full cell assembled with the cathode material.

[0049] As shown in Table 1, the full cell assembled using the sulfate-based cathode material prepared in Example 1 exhibits higher initial discharge capacity and capacity retention after 50 cycles compared to the full cell assembled using the sulfate-based cathode material prepared in Comparative Example 1. Furthermore, the pouch cell in Comparative Example 1 shows significantly more gas expansion than that in Example 1 (visible to the naked eye and very intuitive). This is because the addition of polyaniline allows the sulfate ions released from Na2Mn(SO4)2 to undergo anion doping with the conductive polyaniline. Since the sulfate ions in Na2Mn(SO4)2 are less likely to form SO2 gas, it is more stable in the electrochemical reaction and does not produce excess gas, thus exhibiting higher cycle performance.

[0050] As shown in Table 1, the full cell assembled using the sulfate-based cathode material prepared in Example 2 exhibits higher initial discharge capacity and capacity retention after 50 cycles compared to the full cell assembled using the sulfate-based cathode material prepared in Comparative Example 2. Furthermore, the pouch cell in Comparative Example 2 shows more significant gas expansion than that in Example 2. This is because the hydrothermal method provides a high-pressure environment, allowing the sulfate ions released from Na2Mn(SO4)2 to anion-dopantize with conductive polyaniline, preventing sulfate ion reduction and resulting in higher cycle performance.

[0051] As shown in Table 1, the full cells assembled using the sulfate-based cathode material prepared in Example 3 exhibit higher initial discharge capacity and capacity retention after 50 cycles compared to the full cells assembled using the sulfate-based cathode materials prepared in Comparative Examples 3 and 4. This is because a suitable acidic environment allows polyaniline to maintain its conductive doped structure and suppress Mn. 2+ Hydrolysis and precipitation occur simultaneously; the redox potential is also regulated to ensure the smooth occurrence of the redox reaction and to provide anions to achieve charge balance. Too low or too high a pH value is detrimental to the reaction.

[0052] As shown in Table 1, the full cell assembled using the sulfate-based cathode material prepared in Example 4 exhibits higher initial discharge capacity and capacity retention after 50 cycles compared to the full cells assembled using the sulfate-based cathode materials prepared in Comparative Examples 5 and 6. This is because below 250℃, the crystallinity is poor, resulting in numerous defects; polyaniline is only physically adsorbed, leading to weak bonding and easy detachment; and the reaction kinetics are insufficient, resulting in incomplete recombination. Above 300℃, polyaniline undergoes thermal decomposition and dedoping, destroying the conductive structure; Na2Mn(SO4)2 easily decomposes to generate impurity phases; excessive grain growth deteriorates the electrochemical performance of the material, and the pressure inside the reactor becomes excessive. Only at temperatures between 250 and 300℃ can the matrix crystallization and polyaniline strong coating be balanced, while simultaneously suppressing thermal decomposition and phase transformation, resulting in a composite material with excellent performance.

[0053] As shown in Table 1, the full cell assembled using the sulfate-based cathode material prepared in Example 5 exhibits higher initial discharge capacity and capacity retention after 50 cycles compared to the full cells assembled using the sulfate-based cathode materials prepared in Comparative Examples 7 and 8. This is because insufficient polyaniline makes it difficult to construct a complete conductive network, leading to increased manganese dissolution; excessive polyaniline results in an overly thick coating layer, hindering ion transport and reducing the specific capacity of the composite material. Only when the mass ratio of Na2Mn(SO4)2 to conductive polyaniline is controlled at 40:(2-3) can conductive polyaniline form a thin and continuous conductive protective layer on the surface of the active particles, thus constructing an effective conductive network, reducing charge transfer impedance, suppressing manganese dissolution, and not significantly hindering sodium ion migration.

[0054] As shown in Table 1, the full cell assembled using the sulfate-based cathode material prepared in Example 5 exhibits higher initial discharge capacity and capacity retention after 50 cycles compared to the full cells assembled using the sulfate-based cathode materials prepared in Comparative Examples 9 and 10. This is because a lower weight-average molecular weight results in shorter molecular chains, poor continuity of the conductive network, and insufficient mechanical strength of the coating, making it prone to cracking and dissolution of small-molecule polyaniline during repeated sulfate insertion and extraction. Conversely, an excessively high molecular weight exacerbates polymer chain entanglement, reduces ion transport channels within the polymer, increases the resistance to divalent sulfate migration, induces ion retention, and inhibits reversible doping reactions. Only polyaniline with a weight-average molecular weight of approximately 65,000 can balance continuous conductive pathways, moderate chain entanglement, and film toughness, ensuring smooth and reversible sulfate migration and stabilizing the electrode interface structure.

Claims

1. A method for preparing a sulfate-based sodium ion cathode material, characterized in that, Includes the following steps: (1) Na2Mn(SO4)2 and conductive polyaniline were mixed and then dry ball milled under vacuum to obtain a mixed powder; (2) Add the mixed powder into an alumina corundum ceramic-lined hydrothermal reactor containing N-methylpyrrolidone solvent, adjust the pH value to 3-4, and then place it in a muffle furnace for hydrothermal reaction at 250-300℃. (3) The powder obtained after the hydrothermal reaction is washed with sodium carbonate solution and then vacuum dried to obtain sulfate sodium ion cathode material; In step (1), the mass ratio of Na2Mn(SO4)2 to conductive polyaniline is 40:(2-3); the weight-average molecular weight of conductive polyaniline is 60,000-70,000.

2. The method for preparing the sulfate-based sodium ion cathode material according to claim 1, characterized in that, In step (1), a planetary ball mill is used for dry ball milling. The grinding balls are agate balls, the ball-to-material mass ratio is (10-12):1, the ball milling speed is 150-300 rpm, and the ball milling time is 3-5 h.

3. The method for preparing the sulfate-based sodium ion cathode material according to claim 1, characterized in that, In step (2), the hydrothermal reaction takes 6 to 10 hours.

4. The method for preparing the sulfate-based sodium ion cathode material according to claim 1, characterized in that, In step (2), the heating rate of the hydrothermal reaction is 1 to 3 °C / min.

5. The method for preparing the sulfate-based sodium ion cathode material according to claim 1, characterized in that, In step (2), the amount of N-methylpyrrolidone added accounts for 60-70% of the volume of the hydrothermal reactor lined with alumina corundum ceramic.

6. The method for preparing the sulfate-based sodium ion cathode material according to claim 1, characterized in that, In step (2), citric acid is used to adjust the pH value.

7. The method for preparing the sulfate-based sodium ion cathode material according to claim 1, characterized in that, In step (3), the concentration of the sodium carbonate solution is 1 to 3 mol / L.

8. The method for preparing the sulfate-based sodium ion cathode material according to claim 1, characterized in that, In step (3), the vacuum drying temperature is 90-120℃ and the drying time is 10-12h.

9. A sulfate-based sodium ion cathode material, characterized in that, It is prepared by the preparation method according to any one of claims 1 to 8.

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