A preparation method of a carbon-coated sodium vanadium phosphate cathode material

Abstract

The present invention discloses a method for preparing a carbon-coated sodium vanadium phosphate cathode material. The preparation steps include first uniformly mixing a sodium source, a vanadium source, a phosphorus source, a composite carbon source, and a solvent according to a target stoichiometric ratio, and sand-milling the mixture to obtain a slurry; then spray-drying the slurry to obtain a dry powder; and then sintering the dry powder under vacuum to obtain a product. The composite carbon source includes polymaleic acid and a carbohydrate carbon source. By combining carbon source compounding, spray drying, and vacuum sintering, the present invention can obtain a carbon-coated sodium vanadium phosphate cathode material with high particle size reliability, high compaction density, and excellent electrochemical performance.

Description

Technical Field

[0001] The present invention belongs to the technical field of new energy batteries, and in particular relates to a method for preparing a carbon-coated sodium vanadium phosphate positive electrode material for sodium batteries. Background Art

[0002] In recent years, driven by increasing energy demand and environmental concerns, renewable energy has experienced rapid development. However, renewable energy sources are generally intermittent (such as wind, tidal, and solar energy), requiring suitable energy storage devices. Therefore, rechargeable secondary batteries have become the preferred choice. Since 1991, lithium-ion batteries have been widely used in energy storage and power equipment. However, due to the limited reserves and high price of lithium resources, the search for alternatives to lithium-ion batteries has become a research focus in the battery industry. Compared to lithium, sodium is more abundant in the Earth's crust, and sodium ions and lithium ions have similar chemical properties, making sodium-ion batteries the most likely alternative to lithium-ion batteries.

[0003] Cathode materials are essential components of sodium-ion batteries, primarily including layered oxides, polyanionic compounds, Prussian blue analogs, and organic cathode materials. Polyanionic compounds are a hot topic in sodium-ion battery cathode materials due to their stable structure, excellent performance, and high operating voltage. Among the various types of polyanionic cathode materials, sodium superion conductor (NASICON) phosphates have become a key research focus for sodium-ion battery cathode materials due to their excellent sodium ion conductivity and stable structure. Sodium vanadium phosphate (NVP), one of the materials with a NASICON structure, is a preferred choice for industrial mass production due to its simple preparation process, high reversible capacity, and high operating voltage.

[0004] Carbon coating is the most commonly used method to improve the electrochemical performance of materials, especially for sodium vanadium phosphate. Compared with other polyanionic compounds (such as sodium iron phosphate and sodium iron sulfate), although the introduction of vanadium brings higher operating voltage, ionic conductivity, and structural stability, the unique surrounding structure of vanadium ions in phosphorus-oxygen tetrahedrons makes sodium vanadium phosphate's electronic conductivity extremely low, and it is prone to large polarization under cycling or high-rate conditions. In addition, the performance of the carbon coating layer varies, and the degree of bonding between the carbon layer and the material needs to be improved. The carbon layer may peel off during the material pulverization process. Poor carbon coating also leads to defects such as poor particle size distribution and low compaction density during the particle dispersion and electrode manufacturing process when sodium vanadium phosphate is used as a positive electrode material, resulting in poor electrical performance. These problems have greatly limited the performance improvement of sodium vanadium phosphate. Summary of the Invention

[0005] The present invention addresses the problems in the prior art and the poor carbon coating performance of sodium vanadium phosphate positive electrode materials. A method for preparing a carbon-coated sodium vanadium phosphate positive electrode material by combining carbon source compounding, spray drying and negative pressure sintering is proposed. The carbon-coated sodium vanadium phosphate positive electrode material with high particle size carbon coating reliability, high compaction density and good electrochemical performance can be obtained.

[0006] The method for preparing the carbon-coated sodium vanadium phosphate positive electrode material of the present invention comprises the following steps:

[0007] S1: mixing a sodium source, a vanadium source, a phosphorus source, a composite carbon source, and a solvent in a target stoichiometric ratio, and sand-milling to obtain a slurry; wherein the composite carbon source includes polymaleic acid and a sugar carbon source;

[0008] S2: spray drying the slurry to obtain dry powder;

[0009] S3: Sintering the dried powder under vacuum negative pressure to obtain the product.

[0010] This invention uses polymaleic acid and a carbohydrate carbon source as a composite carbon source for the preparation of a carbon coating layer for sodium vanadium phosphate. The carbohydrate carbon source, with its polyhydroxy aldehyde or polyhydroxy ketone cyclic structure, not only provides excellent hydrophilic stabilization, facilitating the formation of a good spray slurry and promoting adhesion to other raw material surfaces, but also ensures that the resulting carbon layer has a suitable spatial structure. Polymaleic acid can be used as both a carbon source and a dispersant or additive. It has abundant carboxyl functional groups and is easily adsorbed on the particle surface, causing electrostatic repulsion between the particles and forming a film on the particle surface. This prevents direct contact between the particles and promotes the existence of independent particles in the dispersed system. Furthermore, the unique carboxyl topological structure of polymaleic acid can produce anhydride groups (-C(O)OC(O)-), which have a chelating effect with vanadium ions. Its combination with a carbohydrate carbon source not only enhances the network crosslinking density between sodium vanadium phosphate and the carbon coating from an atomic structure perspective, improving electronic conductivity and reducing vanadium-rich impurities formed by vanadium aggregation, but also synergizes with the sugar hydroxyl groups to promote dispersion, improving the adsorption capacity of the carbohydrate carbon source on the particle surface, reducing carbohydrate carbon source aggregation, and improving the distribution of the carbon source and particles. In the subsequent spray drying process, this reduces the formation of larger particles due to aggregation of the material and avoids wall sticking during spray drying, allowing the material to be dried as small carbon source-coated particles to become a dry powder.

[0011] Subsequent sintering is further combined with negative pressure sintering. This negative pressure allows gases like carbon dioxide and water generated during the carbothermal reduction process to migrate to the exterior of the material due to the pressure. Under this pressure, the resulting pores are reduced and restructured, optimizing the density of the carbon coating, resulting in a higher compaction density and a smaller specific surface area. The combination of a composite carbon source ensures that the spray-dried particles possess a refined particle size and a carbon matrix spatial structure suitable for negative pressure sintering. This facilitates gas diffusion and suitable pore construction during the carbothermal reduction process, resulting in superior reaction kinetics.

[0012] In some embodiments, the sodium source can be selected from sodium salts that do not introduce other impurity elements, including but not limited to one or more of NaOH, hydrated NaH2PO4, Na2CO3, and sodium citrate.

[0013] In some embodiments, the vanadium source can be selected from vanadium-containing compounds that do not introduce other impurity elements, including but not limited to one or more of V2O5, VO2, and NH4VO3.

[0014] In some embodiments, the phosphorus source can be selected from phosphoric acid or phosphates that do not introduce other impurity elements, including but not limited to one or more of H3PO4, NH4H2PO4, hydrated NaH2PO4, and (NH4)3PO4.

[0015] In some embodiments, the carbohydrate carbon source includes but is not limited to one or more of monosaccharides (such as glucose and fructose), disaccharides (such as lactose and maltose), polysaccharides (such as starch and cellulose), and sugar alcohols (such as mannitol and sorbitol).

[0016] In some embodiments, the polymaleic acid carbon source includes but is not limited to one or more of polymaleic acid, polymaleic anhydride (PMAH), styrene-maleic anhydride copolymer (SMA), PEG-grafted styrene-maleic anhydride alternating copolymer (P((PEG-g-St)-alt-MA)), etc., with polymaleic acid being preferred.

[0017] In some embodiments, in the composite carbon source, the mass ratio of the polymaleic acid carbon source to the sugar carbon source is 1:(0.5-5), preferably 1:(1-2).

[0018] In some embodiments, in the slurry, the theoretical feed mass ratio of the composite carbon source to sodium vanadium phosphate is 1: (3-10), preferably 1: (4-8), wherein the theoretical feed mass of sodium vanadium phosphate is the maximum production of sodium vanadium phosphate obtained by theoretical calculation of the feed amounts of sodium source, vanadium source, and phosphorus source (ignoring raw material losses and miscellaneous items).

[0019] In some schemes, in the slurry, the molar ratio of the sodium source (calculated as Na), the vanadium source (calculated as V), and the phosphorus source (calculated as P) is (3~3.5):(1.1~2):(3~3.5). An appropriate excess of Na and P is beneficial to saving the precious metal V and effectively recovering the unreacted raw sodium source and phosphorus source.

[0020] In the absence of conflict, two or more of the sodium source, vanadium source, phosphorus source, and composite carbon source can be the same substance. For example, hydrated NaH2PO4 can be used as both a sodium source and a phosphorus source.

[0021] In some embodiments, the molecular weight of the polymaleic acid carbon source is 400-800, which can provide better dispersion, coating and chelation effects.

[0022] The solvent of the present invention is not limited. As a general example, water is selected as the solvent because it has a greater cost advantage.

[0023] In some embodiments, the solid content of the slurry in step S1 can be optimized to 20%-40%, which is more conducive to sand milling and spray drying to form suitable dry particles.

[0024] In some embodiments, the particle size D50 of the slurry is controlled to be 0.1-0.5 μm after sand grinding, which is conducive to forming dry particles of appropriate size during the spray drying process.

[0025] In some solutions, the spray drying peristaltic pump frequency is 15-40 rpm / min, the spray drying air inlet temperature is 150°C-200°C, and the air outlet temperature is 75°C-100°C. Appropriately optimized spray drying conditions are conducive to improving spray drying efficiency.

[0026] In some schemes, the vacuum sintering temperature is 700°C~850°C, and the vacuum degree is ≤0.1 MPa, which is conducive to the full carbonization of the carbon source and the formation of a stable coating layer, and appropriate pore adjustment.

[0027] The present invention provides a sodium ion battery using the carbon-coated sodium vanadium phosphate prepared by the above method as the positive electrode active material. Furthermore, the carbon-coated sodium vanadium phosphate can account for 60% to 95% by weight of the positive electrode.

[0028] Compared with the existing technology, the present invention adopts polymaleic acid and sugar carbon sources as composite carbon sources, and combines spray drying with negative pressure sintering. Through the unique dispersion and chelation effect of the composite carbon source and the influence on the refinement of spray drying particles, the negative pressure sintering process is well pore regulated, the reaction kinetics are optimized, and the formation of a densified cross-linked structure of the carbon coating layer is achieved. The prepared carbon-coated sodium vanadium phosphate positive electrode material has significant advantages in terms of optimized specific surface area, increased compaction density, and increased carbon coating reliability. At the same time, the yield and electrical properties are also optimized and improved. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] In order to more clearly illustrate the technical solutions in the embodiments of the present application, the following briefly introduces the drawings required for use in the description of the embodiments. Obviously, the drawings described below are only some embodiments of the present application. For ordinary technicians in this field, other drawings can be obtained based on these drawings without any creative work.

[0030] Figure 1 This is the XRD spectrum of NVP prepared in Example 1.

[0031] Figure 2 Charge and discharge curves of the sodium ion half-cell assembled in Example 1 at room temperature (25°C) and a current density of 0.1 C.

[0032] Figure 3 Charge and discharge curves of the sodium ion half-cell assembled in Example 1 at room temperature (25°C) and a current density of 0.5 C. DETAILED DESCRIPTION

[0033] The following will be combined with the accompanying drawings in the embodiments of this application to clearly and completely describe the technical solutions in the embodiments of this application. Obviously, the embodiments described are only part of the embodiments of this application, not all of the embodiments. Based on the embodiments of this application, all other embodiments obtained by ordinary technicians in this field without making creative efforts are within the scope of protection of this application.

[0034] The chemical raw materials involved in the following examples and comparative examples are all prior art and are commercially available. The experimental devices, test devices, etc. involved in the following examples and comparative examples are all conventional devices in the art and are not particularly limited.

[0035] Example 1

[0036] Weigh 891.43 g of pure water, 29.22 g of sucrose, 24 g of polymaleic acid (molecular weight 400), 155.3 g of vanadium phosphate (V₂O₅), and 415.51 g of sodium bicarbonate (NaH₂PO₄·2H₂O) in that order and add them to a sand mill. Rinse the remaining material with 222.9 g of water. After sand milling, the D₁₀ values were 0.13 ± 0.05 μm, 0.23 ± 0.05 μm, and 1.0 ± 0.15 μm, resulting in a homogeneous slurry.

[0037] The air supply temperature of the spray dryer was set to 180°C. After reaching the required temperature, the slurry was added to the spray dryer using a peristaltic pump at a pump frequency of 25 Hz and an air outlet temperature of 90°C. After spray drying, a dry powder was obtained.

[0038] The spray-dried powder was placed in a quartz glass tube, which was evacuated to a vacuum of 0.1 MPa using a glass tube sealer and sealed. The sealed tube was then sintered in a tube furnace at 750°C for 10 hours at a heating rate of 4°C / min. After sintering, the tube was naturally cooled to room temperature. The tube was opened to obtain the NVP cathode material.

[0039] Preparation of button cells: Take 0.6 g of binder (5% PVDF + NMP) and 0.3 g of SP and stir them in a degassing machine. After stirring, add 2.4 g of NVP and stir again to obtain a uniform slurry. The slurry is evenly coated on the surface of aluminum foil, and the cut pieces are dried to obtain electrode sheets. The battery is assembled and tested using metallic sodium as the negative electrode and 1M NaClO4 (PC + 2% FEC) as the electrolyte.

[0040] Example 2

[0041] The NVP positive electrode material and button battery were prepared respectively by the same preparation method as in Example 1, except that the sucrose in Example 1 was replaced by 33.58 g of glucose.

[0042] Example 3

[0043] The NVP positive electrode material and button battery were prepared respectively by the same preparation method as in Example 1, except that the sucrose in Example 1 was replaced by 30.21 g of starch.

[0044] Example 4

[0045] The NVP positive electrode material and button battery were prepared respectively by the same preparation method as in Example 1, except that the amounts of sucrose and polymaleic acid used in Example 1 were 32.22 g and 21.23 g, respectively.

[0046] Example 5

[0047] The NVP positive electrode material and button battery were prepared respectively by the same preparation method as in Example 1, except that 155.3 g V2O5 and 415.51 g NaH2PO4·2H2O in Example 1 were replaced by 199.72 g NH4VO3, 397.1 g (NH4)3PO4 and 141.15 g Na2CO3.

[0048] Example 6

[0049] The same preparation method as in Example 1 was used to prepare NVP positive electrode material and button battery, except that the sanding medium and sanding time were changed. After sanding, the D10 was 0.11±0.05 μm, the D50 was 0.25±0.05 μm, and the D97 was 1.2±0.15 μm, obtaining a uniform slurry.

[0050] Example 7

[0051] The NVP positive electrode material and button battery were prepared respectively by the same preparation method as in Example 1, except that the spray drying in Example 1 was set to an air supply temperature of 160°C and an air outlet temperature of 80°C.

[0052] Example 8

[0053] The NVP positive electrode material and button battery were prepared respectively by the same preparation method as in Example 1, except that the sintering temperature in Example 1 was set to 700°C.

[0054] Comparative Example 1

[0055] The NVP positive electrode material and button battery were prepared respectively by the same preparation method as in Example 1, except that the dry powder in Example 1 was sintered in a tubular furnace in a nitrogen atmosphere.

[0056] Comparative Example 2

[0057] The NVP positive electrode material and button battery were prepared respectively by the same preparation method as in Example 1, except that the dry powder in Example 1 was sintered in a tube furnace in an argon atmosphere.

[0058] Comparative Example 3

[0059] The NVP positive electrode material and button battery were prepared respectively by the same preparation method as in Example 1, except that polymaleic acid was not added and the amount of sucrose added was changed to 53.21 g.

[0060] Comparative Example 4

[0061] NVP positive electrode material and button battery were prepared respectively by the same preparation method as in Example 1, except that polymaleic acid was not added, the amount of sucrose added was changed to 53.21 g, and the spray-dried powder was sintered in a tubular furnace under a nitrogen atmosphere.

[0062] Comparative Example 5

[0063] NVP positive electrode material and button battery were prepared respectively by the same preparation method as in Example 1, except that polymaleic acid was not added, the amount of sucrose added was changed to 53.21 g, and the spray-dried powder was sintered in a tube furnace under argon atmosphere.

[0064] Comparative Example 6

[0065] The NVP positive electrode material and button battery were prepared respectively by the same preparation method as in Example 1, except that the polymaleic acid in the example was replaced by 48 g PEG-200.

[0066] Comparative Example 7

[0067] The NVP positive electrode material and button battery were prepared respectively by the same preparation method as in Example 1, except that the polymaleic acid in the example was replaced with 48 g PEG-200, and the spray-dried powder was sintered in a tubular furnace under a nitrogen atmosphere.

[0068] Comparative Example 8

[0069] The NVP positive electrode material and button cell were prepared respectively by the same preparation method as in Example 1, except that the polymaleic acid in the example was replaced with 48 g PEG-200, and the spray-dried powder was sintered in a tube furnace under an argon atmosphere.

[0070] Comparative Example 9

[0071] The NVP positive electrode material and button battery were prepared respectively by the same preparation method as in Example 1, except that the sucrose and polymaleic acid in the example were replaced by 57.57 g of glucose.

[0072] The above embodiments of the present invention are tested for performance indicators and test results are obtained.

[0073] Table 1 shows the test results of Examples 1-8 of the present invention. It can be seen that the carbon-coated sodium vanadium phosphate positive electrode material prepared by combining carbon source compounding, spray drying and negative pressure sintering in the present invention exhibits a smaller specific surface area and a higher compaction density under good carbon coating. This is attributed to the optimized pore reconstruction during the preparation process. The material also exhibits a higher first efficiency (25°C, 0.5C charge and discharge) and cycle capacity retention rate (25°C, 0.5C charge and discharge) in the battery positive electrode, indicating that the electrical performance of sodium vanadium phosphate has been greatly improved under the multiple effects of synergistic optimization of polymaleic acid and sugar carbon sources, chelation, and spray particle refinement. Figure 1 The XRD spectrum of NVP prepared in Example 1 is shown, and it can be seen that the present invention can obtain a sodium vanadium phosphate positive electrode material with a well-developed structure. Figure 2 、 Figure 3 The charge and discharge curves of Example 1 at different current densities are shown, which shows that the carbon-coated sodium vanadium phosphate positive electrode material prepared by the present invention can exert good electrical properties.

[0074] Table 1

[0075]

[0076] In order to further demonstrate the advantages of the preparation method of the present invention, comparative examples 1-9 were also designed, and the test results are listed in Table 2 and Table 3.

[0077] Table 2

[0078]

[0079] Table 3

[0080]

[0081] As shown in Example 1 relative to Comparative Examples 1-2, thanks to the compounded carbon source and the refined particles of spray drying, the negative pressure sintering process adopted in the present invention enables the pores of the material to be optimized and reconstructed during the carbon thermal reduction process. Compared with general atmosphere sintering, it exhibits a lower specific surface area and a higher compaction density, and the product particle size distribution is more concentrated (D10, D50, D90), indicating that the density of the carbon coating also brings about an improvement in reliability and reduces the shedding of the coated carbon, all of which lead to better electrical performance.

[0082] Example 1 demonstrates the important role of polymaleic acid as a composite carbon source, compared to Comparative Examples 3-5 and Comparative Examples 6-8. In the schemes without polymaleic acid (Comparative Examples 3-5), the lack of its dispersion-promoting and chelating effects on the raw material particles not only easily leads to wall sticking, which reduces the yield, but also the lack of chelation leads to poor material coating, making the carbon coating more likely to fall off, further reducing the yield. Furthermore, the network crosslinking density between the sodium vanadium phosphate and the carbon coating is reduced, and the internal resistance of the material is easily increased due to the polarization of vanadium ions during cycling, thereby reducing cycling performance. The impact of not sintering under vacuum in Comparative Examples 3-5 (Comparative Examples 4 and 5) also shows the same trend as Comparative Examples 1-2. When polymaleic acid was replaced with polyethylene glycol (PEG), which also possesses carboxyl groups (Comparative Examples 6-8), while the carbon source's compounding effect was slightly improved due to the introduction of hydroxyl groups, the overall improvement was less pronounced due to the lack of polymaleic acid's unique carboxyl topological structure. Furthermore, the linear nature of PEG itself may have a different effect on the gas discharge channels generated during sintering, affecting pore formation, resulting in a significant difference in cycling performance. Comparative Example 9 also demonstrates that changing the carbohydrate carbon source still fails to achieve comparable performance results to those in this case.

[0083] The various technical features of the above-described embodiments can be combined arbitrarily. In order to make the description concise, not all possible combinations of the various technical features in the above-described embodiments are described. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of this specification. Although the embodiments of the present invention have been shown and described above, it can be understood that the above-described embodiments are exemplary and cannot be understood as limitations of the present invention. Those skilled in the art can change, modify, replace and deform the above-described embodiments within the scope of the present invention. In addition, those skilled in the art can combine and combine the different embodiments or examples described in this specification and the features of different embodiments or examples, unless they are contradictory.

Claims

1. A method for preparing a carbon-coated sodium vanadium phosphate positive electrode material, comprising the following steps: S1: Mix the sodium source, vanadium source, phosphorus source, composite carbon source and solvent uniformly according to the target stoichiometric ratio, and sand grind to obtain a slurry; wherein, Complex carbon sources include polymaleic acid and sugar carbon sources; S2: spray drying the slurry to obtain dry powder; S3: sintering the dried powder under vacuum to obtain the product; In the composite carbon source, the mass ratio of the polymaleic acid carbon source to the sugar carbon source is 1: (0.5-5); In the slurry, the theoretical feed mass ratio of the composite carbon source to sodium vanadium phosphate is 1:(3-10); After sand grinding, the particle size D50 of the slurry is controlled to be between 0.1 and 0.5 μm; The temperature of vacuum sintering is 700℃~850℃, and the vacuum degree is ≤0.1 Mpa.

2. The method for preparing the carbon-coated sodium vanadium phosphate positive electrode material according to claim 1, characterized in that: The sodium source is selected from one or more of NaOH, hydrated NaH2PO4, Na2CO3, and sodium citrate; And / or, the vanadium source is selected from one or more of V2O5, VO2, and NH4VO3; And / or, the phosphorus source is selected from one or more of H3PO4, NH4H2PO4, hydrated NaH2PO4, (NH4)3PO4; And / or, the carbohydrate carbon source is selected from one or more of monosaccharides, disaccharides, polysaccharides and sugar alcohols; And / or, the polymaleic acid carbon source is selected from one or more of polymaleic acid, polymaleic anhydride, styrene-maleic anhydride copolymer, and PEG-grafted styrene-maleic anhydride alternating copolymer; And / or, the solvent is water.

3. The method for preparing the carbon-coated sodium vanadium phosphate cathode material according to claim 1, characterized in that: In the composite carbon source, the mass ratio of the polymaleic acid carbon source to the sugar carbon source is 1:(1-2).

4. The method for preparing the carbon-coated sodium vanadium phosphate cathode material according to claim 1, characterized in that: In the slurry, the theoretical feed mass ratio of the composite carbon source to the sodium vanadium phosphate is 1:(4-8).

5. The method for preparing the carbon-coated sodium vanadium phosphate cathode material according to claim 1, characterized in that: In the slurry, the molar ratio of the sodium source (calculated as Na), the vanadium source (calculated as V), and the phosphorus source (calculated as P) is (3-3.5):(1.1-2):(3-3.5).

6. The method for preparing the carbon-coated sodium vanadium phosphate cathode material according to claim 1, characterized in that: The molecular weight of the polymaleic acid carbon source is 400-800.

7. The method for preparing the carbon-coated sodium vanadium phosphate cathode material according to claim 1, characterized in that: The solid content of the slurry in step S1 is 20%-40%; And / or, the spray drying peristaltic pump frequency is 15~40 rpm / min, the spray drying air inlet temperature is 150℃~200℃, and the air outlet temperature is 75℃~100℃.

8. A sodium ion battery comprising the carbon-coated sodium vanadium phosphate prepared by the method according to any one of claims 1 to 7 as a positive electrode active material.

9. The sodium ion battery according to claim 8, characterized in that Carbon-coated sodium vanadium phosphate accounts for 60%-95% by mass in the positive electrode material of sodium ion batteries.

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