Preparation method of sodium ion battery positive electrode material with high water oxygen stability

Abstract

The invention discloses a preparation method of a sodium ion battery positive electrode material with high water-oxygen stability, which comprises the following steps: S1, preparation of a precursor: mixing an iron source, a sodium source and a phosphorus source, and then preparing an NFPP precursor; s2, organic carbon source mixing treatment: adding an organic carbon source into the NFPP precursor, and uniformly mixing to obtain NFPP (at) C intermediate precursor powder; s3, depositing a polydimethylsiloxane coating: dispersing the NFPP-coated C intermediate in an ethanol solution for ultrasonic dispersion, respectively adding a hydroxyl end-capping reagent, a cross-linking agent and a catalyst, and uniformly mixing, so that a polydimethylsiloxane layer is deposited on the surface of NFPP particles in situ, and an NFPP-coated PDMS precursor is obtained; and S4, high-temperature sintering: carrying out high-temperature sintering under protection to obtain the final NFPP-coated PDMS composite positive electrode material. The preparation method of the sodium ion battery positive electrode material with high water-oxygen stability has the characteristics of high water-oxygen stability, low side reaction and high process controllability.

Description

Technical Field

[0001] This invention relates to the field of sodium-ion battery technology, specifically to a method for preparing a sodium-ion battery cathode material with high water and oxygen stability. Background Technology

[0002] Sodium-ion batteries, as an emerging energy storage technology, have become an important supplement to lithium-ion batteries due to the abundance and wide distribution of sodium resources, low cost, and excellent low-temperature performance. In recent years, with the continuous expansion of the global energy storage market, sodium-ion batteries have gradually achieved commercial applications in areas such as grid frequency regulation, distributed energy storage, and low-speed electric vehicles. It is projected that by 2030, the sodium-ion battery industry will occupy a more important position in the large-scale energy storage and power battery market. The core lies in the research and development of cathode materials, especially iron-based polyanion cathodes, which are considered one of the most promising directions for large-scale energy storage applications due to their stable structure, high safety, and long cycle life.

[0003] NFPP, a composite phosphate cathode material, is a typical iron-based polyanionic compound that combines the structural units of phosphate and pyrophosphate. Its crystal framework is composed of FeO6 octahedra and PO4 and P2O7 polyhedra, which can provide multidimensional Na⁺ diffusion channels, exhibiting high specific capacity, stable structure and excellent safety.

[0004] Meanwhile, the main elements in NFPP materials, Na, Fe, P, and O, are all abundant and environmentally friendly, possessing low cost and commercial viability. However, this material is prone to degradation under air and humidity conditions: on the one hand, the surface reacts with water and oxygen to generate byproducts such as NaOH and Na2CO3, accompanied by Na⁺ / H⁺ exchange, leading to lattice distortion and particle breakage; on the other hand, surface Fe²⁺ is easily oxidized and dissolved under the action of oxygen, causing the formation of an amorphous layer and significantly increasing the interfacial resistance; during long-term cycling, these changes will exacerbate the side reactions at the electrode / electrolyte interface, causing the electrochemical performance to gradually decline.

[0005] To improve the air and humidity stability of NFPP, existing research mainly employs methods such as carbon coating, metal oxide coating, and heteroelement doping, but all of these methods have certain limitations:

[0006] For example, carbon coating can effectively improve electronic conductivity, but its barrier effect against moisture and oxygen is limited, and it is prone to gradual degradation in humid and hot environments.

[0007] Metal oxide coatings (such as Al2O3, ZrO2, etc.) can improve interface stability to some extent, but excessively thick coatings can hinder Na⁺ migration, leading to a decrease in rate performance.

[0008] Doping with metallic elements can improve lattice stability, but it cannot fundamentally prevent corrosion from air and humidity. Summary of the Invention

[0009] The purpose of this invention is to provide a method for preparing a sodium-ion battery cathode material with high water and oxygen stability, characterized by high water and oxygen stability, low side reactions, and strong process controllability.

[0010] This invention can be achieved through the following technical solutions:

[0011] The present invention discloses a method for preparing a sodium-ion battery cathode material with high water and oxygen stability, comprising the following steps:

[0012] S1. Preparation of precursor: Iron source, sodium source and phosphorus source are mixed and then NFPP precursor is prepared;

[0013] S2: Organic carbon source mixing treatment: Add organic carbon source to NFPP precursor and mix evenly to obtain NFPP@C intermediate precursor powder;

[0014] S3. Polydimethylsiloxane coating deposition: After ultrasonically dispersing the NFPP@C intermediate in an ethanol solution, the hydroxyl end-capping agent, crosslinking agent, and catalyst are added and mixed evenly to deposit a polydimethylsiloxane layer on the surface of the NFPP particles in situ, thus obtaining the NFPP@PDMS precursor.

[0015] S4. High-temperature sintering: After high-temperature sintering under protection, the final NFPP@PDMS composite cathode material is obtained.

[0016] Furthermore, in step S3, the hydroxyl end-capping agent is one or more of dihydroxy polydimethylsiloxane, diphenylsiloxane copolymer, trifluoropropylmethylsiloxane copolymer, and polydimethylsiloxane.

[0017] Furthermore, in step S3, the crosslinking agent is one or more of the following: tetraethyl orthosilicate, methyl orthosilicate, methyltrimethoxysilane, phenyltrimethoxysilane, polydimethylsiloxane, polymethylhydrosiloxane, and 3-aminopropyltriethoxysilane.

[0018] Furthermore, in step S3, the catalyst is one or more of dibutyltin dilaurate, dibutyltin diacetate, tetraisopropyl titanate, and isopropanol chloroplatinate.

[0019] Furthermore, in step S3, relative to the mass of the NFPP@C precursor, the amount of hydroxyl capping agent added is 0.5-5.0 wt%, the amount of crosslinking agent added is 0.2-3.0 wt%, and the amount of catalyst added is 0.01-0.5 wt%.

[0020] In this invention, all the above addition amounts are relative to the mass of the NFPP@C intermediate. Specifically, the addition amount of the hydroxyl end-capping agent is 0.5-5 wt%. As the source of the PDMS main chain, the amount of the hydroxyl end-capping agent determines the continuity and hydrophobicity of the final coating. When the addition amount is <0.5 wt%, the PDMS chain segments are insufficient, making it difficult to form a continuous coverage; when it is >5 wt%, an excessively thick organic layer is easily formed, hindering ion migration. The addition amount of the crosslinking agent is 0.2-3.0 wt%. The crosslinking agent determines the crosslinking density and compactness of the PDMS. If the amount of the crosslinking agent is too low, the coating is loose and easily penetrated by the electrolyte. If the amount of the crosslinking agent is too high, a highly crosslinked hard shell will be formed, reducing interfacial ion transport. The addition amount of the catalyst is 0.01-0.5 wt%. The catalyst only plays a role in promoting the polycondensation reaction. If the amount of the catalyst is <0.01 wt%, the reaction is incomplete; if the amount of the catalyst is >0.5 wt%, metal residues are easily introduced, affecting interfacial stability. Therefore, an appropriate amount of addition is beneficial for forming a continuous, dense but not overly barrier PDMS nanolayer.

[0021] Furthermore, in step S4, the protective atmosphere is nitrogen or argon, the sintering temperature is 580-620℃, and the heating rate is 1-5℃ / min. -1 .

[0022] In this invention, regarding sintering temperature, if the sintering temperature is <580 ℃, NFPP crystallization is incomplete, the carbon layer conductive network is not fully formed, and PDMS crosslinking is insufficient, resulting in low initial capacity and poor cycle stability. If the sintering temperature is >620 ℃, the PDMS layer undergoes excessive decomposition or even failure, organic groups are destroyed, hydrophobicity decreases, and NFPP grain coarsening may occur, leading to accelerated capacity decay after water absorption. Therefore, 580–620 ℃ is the optimal sintering window that balances NFPP crystal integrity and conductive carbon formation. Regarding heating rate, 1–5 ℃ / min is acceptable. If <1 ℃·min⁻¹, the process efficiency is low, the heating rate is too fast, PDMS decomposes rapidly, and cracks or coating peeling are easily formed. 2 ℃ / min exhibits the best repeatability and coating integrity.

[0023] Furthermore, in step S2, the organic carbon source is one or more of glucose, sucrose, fructose, citric acid, oxalic acid, acetic acid, tartaric acid, polyethylene glycol, and polyvinylpyrrolidone, and the amount of the organic carbon source added is 2.0-5.0% of the mass of the NFPP precursor.

[0024] Further, in step S1, the molar ratio of sodium to phosphorus is 1.002-1.014, and the molar ratio of iron to phosphorus is 0.710-0.725. In this invention, the molar ratio range is based on a comprehensive optimization result that considers suppressing side reactions and impurity phase formation, crystal structure integrity, and electrochemical performance. A sodium-to-phosphorus ratio slightly greater than 1 is used to compensate for Na volatilization during high-temperature sintering, ensuring that Na sites are fully occupied in the crystal structure and preventing sodium-iron inter-site occupation and mixing. If the sodium-to-phosphorus ratio is too high, impurity phases such as sodium iron phosphate and sodium phosphate are easily formed. If the sodium-to-phosphorus ratio is too low, the crystal tends to transform into sodium iron pyrophosphate during sintering, and the number of sodium vacancies increases, which greatly affects the charging capacity. A suitable iron-to-phosphorus ratio is used to ensure the stability of the configuration of FeO6, phosphate, and pyrophosphate. If the iron-to-phosphorus ratio is too high, impurity phases such as Fe2O3 are easily formed. If the iron-to-phosphorus ratio is too low, there are insufficient active sites, the iron plateau is reduced, and the capacity is limited.

[0025] Further, in step S1, the sodium source is one or more of Na2CO3, NaHCO3, NaOH, NaNO3, NaH2PO4, Na2HPO4, Na4P2O7, and NaCl; the iron source is one or more of FeC2O4·2H2O, Fe(NO3)3·9H2O, FeSO4·7H2O, FePO4, FeO, Fe2O3, Fe3O4, and Fe(CH3COO)2; and the phosphorus source is one or more of H3PO4, (NH4)H2PO4, NaH2PO4, Na2HPO4, (NH4)2HPO4, Na4P2O7, K4P2O7, and (NaPO3)6.

[0026] Furthermore, in step S1, the NFPP precursor is prepared by sol-gel method or spray drying method; in step S2, the mixing method is ball milling or stirring. Regarding the preparation method, the sol-gel method has the advantage of uniform composition, which is beneficial for subsequent uniform anchoring of PDMS; the spray drying method is characterized by controllable particle size, suitable for large-scale production; other feasible methods such as co-precipitation, solid-phase method, and hydrothermal method, although inferior to the former two in terms of component uniformity or the number of surface active groups, can be selected based on practical considerations. Regarding the mixing method, ball milling is beneficial for uniform dispersion of the carbon source, forming a continuous conductive network; stirring is suitable for liquid phase systems, with simple processes and good scalability. Considering uniformity, cost, and industrial feasibility, ball milling and stirring are optimal. Other feasible solutions such as ultrasonic dispersion, high-shear mixing, and planetary mixing can be selected according to actual needs to meet the objectives of this invention.

[0027] This invention provides a method for preparing a sodium-ion battery cathode material with high water and oxygen stability, which has the following beneficial effects:

[0028] First, it exhibits high water and oxygen stability. Compared to traditional carbon coatings, polydimethylsiloxane possesses higher chemical inertness and density, maintaining surface stability under air and humidity conditions, thus significantly improving the material's water and oxygen stability. For example, this invention employs a sol-gel method for in-situ deposition of polydimethylsiloxane, which chemically bonds with the functional groups (–OH, –NH2) on the surface of NFPP particles, forming a firmly bonded interface structure. Furthermore, the surface of polydimethylsiloxane can be further modified with organic functionalization to enhance its adhesion to the binder during electrode slurry preparation, thereby improving the overall structural stability and processing performance of the electrode.

[0029] Secondly, the side reactions are low. This invention effectively isolates the penetration of water molecules and oxygen by constructing a nano-polydimethylsiloxane coating layer on the surface of NFPP in situ, thus avoiding adverse reactions such as Na⁺ / H⁺ exchange, by-product (such as Na₂CO₃, NaOH) generation and Fe dissolution.

[0030] Third, the process is highly controllable. The preparation method of this invention balances ion transport performance and cycle stability. In contrast, traditional metal oxide coatings are prone to becoming too thick, hindering Na⁺ diffusion. This invention, by controlling the silicon source hydrolysis rate, stabilizes the polydimethylsiloxane coating at a certain thickness, ensuring both the barrier effect and the diffusion of Na⁺ in the NFPP lattice. Detailed Implementation

[0031] To enable those skilled in the art to better understand the technical solution of the present invention, the product of the present invention will be further described in detail below with reference to embodiments.

[0032] The present invention discloses a method for preparing a sodium-ion battery cathode material with high water and oxygen stability, comprising the following steps:

[0033] S1. Preparation of precursor: Iron source, sodium source and phosphorus source are mixed and then NFPP precursor is prepared;

[0034] S2: Organic carbon source mixing treatment: Add organic carbon source to NFPP precursor and mix evenly to obtain NFPP@C intermediate precursor powder;

[0035] S3. Polydimethylsiloxane coating deposition: After ultrasonically dispersing the NFPP@C intermediate in an ethanol solution, the hydroxyl end-capping agent, crosslinking agent, and catalyst are added and mixed evenly to deposit a polydimethylsiloxane layer on the surface of the NFPP particles in situ, thus obtaining the NFPP@PDMS precursor.

[0036] S4. High-temperature sintering: After high-temperature sintering under protection, the final NFPP@PDMS composite cathode material is obtained.

[0037] Furthermore, in step S3, the hydroxyl end-capping agent is one or more of dihydroxy polydimethylsiloxane, diphenylsiloxane copolymer, trifluoropropylmethylsiloxane copolymer, and polydimethylsiloxane.

[0038] Furthermore, in step S3, the crosslinking agent is one or more of the following: tetraethyl orthosilicate, methyl orthosilicate, methyltrimethoxysilane, phenyltrimethoxysilane, polydimethylsiloxane, polymethylhydrosiloxane, and 3-aminopropyltriethoxysilane.

[0039] Furthermore, in step S3, the catalyst is one or more of dibutyltin dilaurate, dibutyltin diacetate, tetraisopropyl titanate, and isopropanol chloroplatinate.

[0040] Furthermore, in step S3, relative to the mass of the NFPP@C precursor, the amount of hydroxyl capping agent added is 0.5-5.0 wt%, the amount of crosslinking agent added is 0.2-3.0 wt%, and the amount of catalyst added is 0.01-0.5 wt%.

[0041] Furthermore, in step S4, the protective atmosphere is nitrogen or argon, the sintering temperature is 580-620℃, and the heating rate is 1-5℃ / min. -1 .

[0042] Furthermore, in step S2, the organic carbon source is one or more of glucose, sucrose, fructose, citric acid, oxalic acid, acetic acid, tartaric acid, polyethylene glycol, and polyvinylpyrrolidone, and the amount of the organic carbon source added is 2.0-5.0% of the mass of the NFPP precursor.

[0043] Furthermore, in step S1, the molar ratio of sodium to phosphorus is 1.002-1.014, and the molar ratio of iron to phosphorus is 0.710-0.725.

[0044] Further, in step S1, the sodium source is one or more of Na2CO3, NaHCO3, NaOH, NaNO3, NaH2PO4, Na2HPO4, Na4P2O7, and NaCl; the iron source is one or more of FeC2O4·2H2O, Fe(NO3)3·9H2O, FeSO4·7H2O, FePO4, FeO, Fe2O3, Fe3O4, and Fe(CH3COO)2; and the phosphorus source is one or more of H3PO4, (NH4)H2PO4, NaH2PO4, Na2HPO4, (NH4)2HPO4, Na4P2O7, K4P2O7, and (NaPO3)6.

[0045] Furthermore, in step S1, the NFPP precursor is prepared by sol-gel method or spray drying method; in step S2, the mixing is carried out by ball milling or stirring.

[0046] Example 1

[0047] This embodiment relates to a sodium-ion battery cathode material with high water and oxygen stability, and its preparation method includes the following steps.

[0048] S1. Preparation of the precursor: Iron, sodium, and phosphorus sources are mixed to obtain the NFPP precursor. Specifically, the molar ratio of sodium to phosphorus is 1.014, and the molar ratio of iron to phosphorus is 0.715. The sodium sources are Na2CO3, NaHCO3, NaOH, NaNO3, and NaH2PO4; the iron sources are FeC2O4·2H2O, Fe(NO3)3·9H2O, and FeSO4·7H2O; the phosphorus sources are H3PO4, (NH4)H2PO4, and NaH2PO4. The NFPP precursor is prepared by the sol-gel method.

[0049] S2: Organic carbon source mixing treatment: An organic carbon source is added to the NFPP precursor and mixed uniformly to obtain NFPP@C intermediate precursor powder. Specifically, the organic carbon source is glucose, sucrose, fructose, citric acid, or oxalic acid, and the amount of the organic carbon source added is 5.0% of the mass of the NFPP precursor.

[0050] S3. Polydimethylsiloxane Coating Deposition: After ultrasonically dispersing the NFPP@C intermediate in an ethanol solution, the hydroxyl end-capping agent, crosslinking agent, and catalyst are added separately and ball-milled until homogeneous, causing an in-situ deposition of a polydimethylsiloxane layer on the surface of the NFPP particles, thus obtaining the NFPP@PDMS precursor. Specifically, the hydroxyl end-capping agent is a copolymer of dihydroxypolydimethylsiloxane and diphenylsiloxane; the crosslinking agent is tetraethyl orthosilicate, methyl orthosilicate, and methyltrimethoxysilane; the catalyst is dibutyltin dilaurate; relative to the mass of the NFPP@C precursor, the amount of hydroxyl end-capping agent added is 5.0 wt%, the amount of crosslinking agent added is 2.0 wt%, and the amount of catalyst added is 0.05 wt%.

[0051] S4. High-Temperature Sintering: The final NFPP@PDMS composite cathode material is obtained after high-temperature sintering under protective atmosphere. Specifically, the protective atmosphere is argon, the sintering temperature is 620℃, and the heating rate is 3℃ / min. -1 .

[0052] Example 2

[0053] This embodiment relates to a sodium-ion battery cathode material with high water and oxygen stability, and its preparation method includes the following steps.

[0054] S1. Preparation of the precursor: Iron, sodium, and phosphorus sources are mixed to obtain the NFPP precursor. Specifically, the molar ratio of sodium to phosphorus is 1.008, and the molar ratio of iron to phosphorus is 0.710. The sodium sources are Na2CO3, Na2HPO4, Na4P2O7, and NaCl; the iron sources are FeSO4·7H2O, FePO4, FeO, Fe2O3, Fe3O4, and Fe(CH3COO)2; the phosphorus sources are H3PO4, (NH4)H2PO4, NaH2PO4, Na2HPO4, and (NaPO3)6. The NFPP precursor is prepared by spray drying.

[0055] S2: Organic carbon source mixing treatment: An organic carbon source is added to the NFPP precursor and mixed uniformly to obtain NFPP@C intermediate precursor powder. Specifically, the organic carbon source is glucose, sucrose, fructose, or polyvinylpyrrolidone, and the amount of the organic carbon source added is 4% of the mass of the NFPP precursor.

[0056] S3. Polydimethylsiloxane Coating Deposition: After ultrasonically dispersing the NFPP@C intermediate in an ethanol solution, the hydroxyl-endogen, crosslinking agent, and catalyst are added and stirred evenly to deposit a polydimethylsiloxane layer on the surface of the NFPP particles in situ, thus obtaining the NFPP@PDMS precursor. Specifically, the hydroxyl-endogen is trifluoropropylmethylsiloxane copolymer or polydimethylsiloxane; the crosslinking agent is phenyltrimethoxysilane or polydimethylsiloxane; the catalyst is tetraisopropyl titanate or isopropanol chloroplatinate; relative to the mass of the NFPP@C precursor, the amount of hydroxyl-endogen added is 3 wt%, the amount of crosslinking agent added is 0.2 wt%, and the amount of catalyst added is 0.5 wt%.

[0057] S4. High-Temperature Sintering: The final NFPP@PDMS composite cathode material is obtained after high-temperature sintering under protective atmosphere. Specifically, the protective atmosphere is nitrogen and argon, the sintering temperature is 600℃, and the heating rate is 1℃ / min. -1 .

[0058] Example 3

[0059] This embodiment relates to a sodium-ion battery cathode material with high water and oxygen stability, and its preparation method includes the following steps.

[0060] S1. Preparation of the precursor: Iron, sodium, and phosphorus sources are mixed to obtain the NFPP precursor. Specifically, the molar ratio of sodium to phosphorus is 1.002, and the molar ratio of iron to phosphorus is 0.725. The sodium sources are Na2CO3, NaHCO3, NaOH, NaNO3, and NaH2PO4; the iron sources are FeC2O4·2H2O, Fe(NO3)3·9H2O, and Fe(CH3COO)2; and the phosphorus sources are (NH4)2HPO4, Na4P2O7, K4P2O7, and (NaPO3)6. The NFPP precursor is prepared by spray drying.

[0061] S2: Organic carbon source mixing treatment: An organic carbon source is added to the NFPP precursor and mixed uniformly to obtain NFPP@C intermediate precursor powder. Specifically, the organic carbon source is glucose, sucrose, fructose, polyethylene glycol, or polyvinylpyrrolidone, and the amount of the organic carbon source added is 2.0% of the mass of the NFPP precursor.

[0062] S3. Polydimethylsiloxane Coating Deposition: After ultrasonically dispersing the NFPP@C intermediate in an ethanol solution, the hydroxyl end-capping agent, crosslinking agent, and catalyst are added separately and ball-milled uniformly to deposit a polydimethylsiloxane layer on the surface of the NFPP particles in situ, thus obtaining the NFPP@PDMS precursor. Specifically, the hydroxyl end-capping agent is a copolymer of dihydroxypolydimethylsiloxane and diphenylsiloxane; the crosslinking agent is polydimethylsiloxane, polymethylhydrosiloxane, and 3-aminopropyltriethoxysilane; the catalyst is dibutyltin dilaurate and isopropanol chloroplatinate; relative to the mass of the NFPP@C precursor, the amount of hydroxyl end-capping agent added is 0.5 wt%, the amount of crosslinking agent added is 3.0 wt%, and the amount of catalyst added is 0.3 wt%.

[0063] S4. High-Temperature Sintering: The final NFPP@PDMS composite cathode material is obtained after high-temperature sintering under protective atmosphere. Specifically, the protective atmosphere is nitrogen or argon, the sintering temperature is 580℃, and the heating rate is 5℃ / min. -1 .

[0064] Example 4

[0065] This embodiment relates to a sodium-ion battery cathode material with high water and oxygen stability, and its preparation method includes the following steps.

[0066] S1. Preparation of the precursor: Iron, sodium, and phosphorus sources are mixed to obtain the NFPP precursor. Specifically, the molar ratio of sodium to phosphorus is 1.002-1.014, and the molar ratio of iron to phosphorus is 0.710-0.725. The sodium source is Na2CO3, NaHCO3, NaOH, or NaNO3; the iron source is FeC2O4·2H2O or Fe(NO3)3·9H2O; and the phosphorus source is H3PO4, (NH4)H2PO4, or NaH2PO4. The NFPP precursor is prepared by spray drying.

[0067] S2: Organic carbon source mixing treatment: An organic carbon source is added to the NFPP precursor and mixed uniformly to obtain NFPP@C intermediate precursor powder. Specifically, the organic carbon source is glucose, sucrose, fructose, or citric acid, and the amount of organic carbon source added is 3% of the mass of the NFPP precursor.

[0068] S3. Polydimethylsiloxane Coating Deposition: After ultrasonically dispersing the NFPP@C intermediate in an ethanol solution, the hydroxyl end-capping agent, crosslinking agent, and catalyst are added separately and ball-milled until homogeneous, causing an in-situ deposition of a polydimethylsiloxane layer on the surface of the NFPP particles, thus obtaining the NFPP@PDMS precursor. Specifically, the hydroxyl end-capping agent is a copolymer of dihydroxypolydimethylsiloxane and diphenylsiloxane; the crosslinking agent is tetraethyl orthosilicate, methyl orthosilicate, methyltrimethoxysilane, and phenyltrimethoxysilane; the catalyst is dibutyltin dilaurate; relative to the mass of the NFPP@C precursor, the amount of hydroxyl end-capping agent added is 2.0 wt%, the amount of crosslinking agent added is 1.0 wt%, and the amount of catalyst added is 0.2 wt%.

[0069] S4. High-Temperature Sintering: The final NFPP@PDMS composite cathode material is obtained after high-temperature sintering under protective atmosphere. Specifically, the protective atmosphere is nitrogen or argon, the sintering temperature is 590℃, and the heating rate is 2℃ / min. -1 .

[0070] Application Example 1

[0071] This embodiment relates to an NFPP@PDMS composite cathode material, the preparation method of which includes the following steps:

[0072] S1. Preparation of precursor: Weigh Na2CO3, FeC2O4·2H2O and (NH4)H2PO4, and mix them according to the sodium-to-phosphorus ratio of 1.005 and the iron-to-phosphorus ratio of 0.722. Add them to deionized water, and stir at 70 °C for 8 h using the sol-gel method. After drying, NFPP precursor powder is obtained.

[0073] S2. Mixing of organic carbon sources: Take the above NFPP precursor powder, add 5 wt% glucose, and ball mill at 500 rpm for 6 h to obtain a uniformly mixed NFPP@C intermediate precursor powder.

[0074] S3. Polydimethylsiloxane coating treatment: NFPP@C precursor powder was dispersed in an ethanol solution, and 3.0% diphenylsiloxane copolymer, 1.5% methyl orthosilicate, and 0.2% tetraisopropyl titanate were added. The mixture was stirred at 60 °C for 4 h to carry out a sol-gel reaction, forming a polydimethylsiloxane coating layer with a thickness of about 5 nm, thus obtaining the NFPP@C@PDMS precursor.

[0075] S4. High-temperature sintering: In a nitrogen atmosphere, the temperature is increased to 590 ℃ at 2 ℃ / min, held for 12 h and then cooled to obtain Na4Fe3(PO4)2P2O7@C composite cathode material.

[0076] Electrochemical performance testing: The above-mentioned cathode material, Super P, and PVDF were mixed in a mass ratio of 80:10:10 to form a slurry, which was then coated onto carbon-coated aluminum foil and vacuum dried at 100 °C for 2 h to prepare an electrode film. The film was punched into a 1.2 mm diameter disc, using metallic sodium as the counter electrode, 1 mol / L NaPF6 (EC:DEC=1:1, v / v) as the electrolyte, and a PP / PE / PP three-layer membrane as the separator, to assemble a CR2016 coin cell. Constant current charge-discharge tests were conducted in the range of 2.0–3.6 V, with a current density of 0.1C (1C=129 mA / g).

[0077] Application Example 2

[0078] This embodiment relates to an NFPP@PDMS composite cathode material, the preparation method of which includes the following steps:

[0079] S1. Preparation of precursor: NaH2PO4, Fe(NO3)3·9H2O and Na2CO3 were used as raw materials, with a sodium-to-phosphorus ratio of 1.004 and an iron-to-phosphorus ratio of 0.713. The NFPP precursor was obtained by spray drying.

[0080] S2. Mixing of organic carbon sources: Take the precursor powder, add 7 wt% sucrose as the carbon source, stir evenly and dry to obtain NFPP@C intermediate.

[0081] S3. Polydimethylsiloxane coating treatment: NFPP@C precursor powder was dispersed in an ethanol solution, and 2.5% polydimethylsiloxane, 1.8% phenyltrimethoxysilane, and 0.3% tetraisopropyl titanate were added. The mixture was stirred at 70°C for 6 h to obtain a polydimethylsiloxane uniformly deposited NFPP@C@PDMS precursor.

[0082] S4. High-temperature sintering: In an argon atmosphere, the temperature is increased to 600 ℃ at 2 ℃ / min and held for 10 h to obtain Na4Fe3(PO4)2P2O7@C composite cathode material.

[0083] Electrochemical performance testing: The above-mentioned cathode material, Super P, and PVDF were mixed in a mass ratio of 80:10:10 to form a slurry, which was then coated onto carbon-coated aluminum foil and vacuum dried at 100 °C for 2 h to prepare an electrode film. The film was punched into a 1.2 mm diameter disc, using metallic sodium as the counter electrode, 1 mol / L NaPF6 (EC:DEC=1:1, v / v) as the electrolyte, and a PP / PE / PP three-layer membrane as the separator, to assemble a CR2016 coin cell. Constant current charge-discharge tests were conducted in the range of 2.0–3.6 V, with a current density of 0.1C (1C=129 mA / g).

[0084] Application Example 3

[0085] This embodiment relates to an NFPP@PDMS composite cathode material, the preparation method of which includes the following steps:

[0086] S1. Preparation of precursors: Using Na2HPO4, Fe2O3 and Na4P2O7 as raw materials, with a sodium-to-phosphorus ratio of 1.006 and an iron-to-phosphorus ratio of 0.721, NFPP precursors were prepared by sol-gel method.

[0087] S2. Mixing of organic carbon sources: Add 3 wt% citric acid as a carbon source, stir and ball mill for 8 h to obtain NFPP@C precursor.

[0088] S3. Polydimethylsiloxane coating treatment: NFPP@C precursor powder was dispersed in an ethanol solution, and 3.0% diphenylsiloxane copolymer, 1.8% phenyltrimethoxysilane, and 0.15% tetraisopropyl titanate were added. The mixture was stirred at 60 °C for 5 h to carry out a sol-gel reaction, forming a polydimethylsiloxane coating layer with a thickness of about 7 nm. The polydimethylsiloxane layer was generated in situ, and the NFPP@C@PDMS precursor was obtained.

[0089] S4. High-temperature sintering: Under nitrogen atmosphere, the temperature is increased to 585 ℃ at 2 ℃ / min and held for 14 h to obtain Na4Fe3(PO4)2P2O7@C composite cathode material.

[0090] Electrochemical performance testing: The above-mentioned cathode material, Super P, and PVDF were mixed in a mass ratio of 80:10:10 to form a slurry, which was then coated onto carbon-coated aluminum foil and vacuum dried at 100 °C for 2 h to prepare an electrode film. The film was punched into a 1.2 mm diameter disc, using metallic sodium as the counter electrode, 1 mol / L NaPF6 (EC:DEC=1:1, v / v) as the electrolyte, and a PP / PE / PP three-layer membrane as the separator, to assemble a CR2016 coin cell. Constant current charge-discharge tests were conducted in the range of 2.0–3.6 V, with a current density of 0.1C (1C=129 mA / g).

[0091] Application Example 4

[0092] This embodiment relates to an NFPP@PDMS composite cathode material, the preparation method of which includes the following steps:

[0093] S1. Preparation of precursor: Fe3O4, NaNO3 and (NH4)2HPO4 were selected as raw materials and the NFPP precursor was synthesized by spray drying with a sodium-to-phosphorus ratio of 1.007 and an iron-to-phosphorus ratio of 0.722.

[0094] S2. Mixing of organic carbon sources: Add 10 wt% polyethylene glycol (PEG) as a carbon source, dissolve and mix under stirring, and dry to obtain NFPP@C.

[0095] S3. Polydimethylsiloxane coating treatment: NFPP@C was dispersed in an ethanol solution, and 1.5% dihydroxypolydimethylsiloxane, 0.2% 3-aminopropyltriethoxysilane, and 0.03% dibutyltin diacetate were added. The mixture was reacted at 70 °C for 6 h to generate a polydimethylsiloxane coating layer, thus obtaining the NFPP@C@PDMS precursor.

[0096] S4. High-temperature sintering: Sintering at 580 °C for 12 h under a nitrogen atmosphere yielded Na4Fe3(PO4)2P2O7@C composite cathode material.

[0097] Electrochemical performance testing: The above-mentioned cathode material, Super P, and PVDF were mixed in a mass ratio of 80:10:10 to form a slurry, which was then coated onto carbon-coated aluminum foil and vacuum dried at 100 °C for 2 h to prepare an electrode film. The film was punched into a 1.2 mm diameter disc, using metallic sodium as the counter electrode, 1 mol / L NaPF6 (EC:DEC=1:1, v / v) as the electrolyte, and a PP / PE / PP three-layer membrane as the separator, to assemble a CR2016 coin cell. Constant current charge-discharge tests were conducted in the range of 2.0–3.6 V, with a current density of 0.1C (1C=129 mA / g).

[0098] Application Example 5

[0099] This embodiment relates to an NFPP@PDMS composite cathode material, the preparation method of which includes the following steps:

[0100] S1. Preparation of precursors: NFPP precursors were synthesized using NaCl, Fe(CH3COO)2 and (NH4)H2PO4 as raw materials, with a sodium-to-phosphorus ratio of 1.003 and an iron-to-phosphorus ratio of 0.723, via a sol-gel method.

[0101] S2. Mixing of organic carbon sources: Add 8 wt% polyvinylpyrrolidone (PVP) as a carbon source, ball mill for 5 h to obtain NFPP@C.

[0102] S3, Polydimethylsiloxane coating treatment: Add 2.0% polydimethylsiloxane, 1.8% phenyltrimethoxysilane, and 0.15% tetraisopropyl titanate, and stir in ethanol solution to form a polydimethylsiloxane coating layer, thus obtaining the NFPP@C@PDMS precursor.

[0103] S4. High-temperature sintering: Sintering at 595 °C for 12 h under nitrogen atmosphere yielded Na4Fe3(PO4)2P2O7@C composite cathode material.

[0104] Electrochemical performance testing: The above-mentioned cathode material, Super P, and PVDF were mixed in a mass ratio of 80:10:10 to form a slurry, which was then coated onto carbon-coated aluminum foil and vacuum dried at 100 °C for 2 h to prepare an electrode film. The film was punched into a 1.2 mm diameter disc, using metallic sodium as the counter electrode, 1 mol / L NaPF6 (EC:DEC=1:1, v / v) as the electrolyte, and a PP / PE / PP three-layer membrane as the separator, to assemble a CR2016 coin cell. Constant current charge-discharge tests were conducted in the range of 2.0–3.6 V, with a current density of 0.1C (1C=129 mA / g).

[0105] Comparative Example 1 (without polydimethylsiloxane coating)

[0106] This embodiment relates to NFPP cathode material, the preparation method of which includes the following steps:

[0107] S1. Preparation of precursor: Weigh Na2CO3, FeC2O4·2H2O and (NH4)H2PO4, mix them in a sodium-to-phosphorus ratio of 1.005 and an iron-to-phosphorus ratio of 0.722, and prepare NFPP precursor by sol-gel method.

[0108] S2. Mixing of organic carbon sources: 5 wt% glucose was added as a carbon source and ball-milled at 500 rpm for 6 h in a planetary ball mill to obtain the NFPP@C precursor.

[0109] S3. High-temperature sintering: Under a nitrogen atmosphere, the temperature was increased to 590 ℃ at 2 ℃ / min and held for 12 h to obtain Na4Fe3(PO4)2P2O7@C composite cathode material.

[0110] Electrochemical performance testing: The above-mentioned cathode material, Super P, and PVDF were mixed in a mass ratio of 80:10:10 to form a slurry, which was then coated onto carbon-coated aluminum foil and vacuum dried at 100 °C for 2 h to prepare an electrode film. The film was punched into a 1.2 mm diameter disc, using metallic sodium as the counter electrode, 1 mol / L NaPF6 (EC:DEC=1:1, v / v) as the electrolyte, and a PP / PE / PP three-layer membrane as the separator, to assemble a CR2016 coin cell. Constant current charge-discharge tests were conducted in the range of 2.0–3.6 V, with a current density of 0.1C (1C=129 mA / g).

[0111] Comparative Example 2 (No carbon source)

[0112] This embodiment relates to NFPP cathode material, the preparation method of which includes the following steps:

[0113] S1. Preparation of precursor: Weigh Na2CO3, FeC2O4·2H2O and (NH4)H2PO4, mix them in a sodium-to-phosphorus ratio of 1.005 and an iron-to-phosphorus ratio of 0.722, and prepare NFPP precursor by sol-gel method.

[0114] S2. Polydimethylsiloxane coating treatment: NFPP@C precursor powder was dispersed in an ethanol solution, and 2.5% polydimethylsiloxane, 1.8% phenyltrimethoxysilane, and 0.3% tetraisopropyl titanate were added. The mixture was stirred at 70°C for 6 h to obtain a polydimethylsiloxane uniformly deposited NFPP@C@PDMS precursor.

[0115] S3. High-temperature sintering: The temperature was increased to 590 ℃ at 2 ℃ / min under a nitrogen atmosphere and held for 12 h to obtain Na4Fe3(PO4)2P2O7@C composite cathode material.

[0116] Electrochemical performance testing: The above-mentioned cathode material, Super P, and PVDF were mixed in a mass ratio of 80:10:10 to form a slurry, which was then coated onto carbon-coated aluminum foil and vacuum dried at 100 °C for 2 h to prepare an electrode film. The film was punched into a 1.2 mm diameter disc, using metallic sodium as the counter electrode, 1 mol / L NaPF6 (EC:DEC=1:1, v / v) as the electrolyte, and a PP / PE / PP three-layer membrane as the separator, to assemble a CR2016 coin cell. Constant current charge-discharge tests were conducted in the range of 2.0–3.6 V, with a current density of 0.1C (1C=129 mA / g).

[0117] Comparative Example 3 (excessively thick polydimethylsiloxane coating)

[0118] This embodiment relates to an NFPP@PDMS composite cathode material, the preparation method of which includes the following steps:

[0119] S1. Preparation of precursor: NaH2PO4, Fe(NO3)3·9H2O and Na2CO3 were used as raw materials, with a sodium-to-phosphorus ratio of 1.004 and an iron-to-phosphorus ratio of 0.723. The NFPP precursor was obtained by spray drying.

[0120] S2. Mixing of organic carbon sources: Take the precursor powder, add 7 wt% sucrose as the carbon source, stir evenly and dry to obtain NFPP@C intermediate.

[0121] S3. Polydimethylsiloxane coating treatment: NFPP@C precursor powder was dispersed in an ethanol solution, and 3.5% polydimethylsiloxane, 1.8% phenyltrimethoxysilane, and 0.15% tetraisopropyl titanate were added. The mixture was stirred at 60 °C for 5 h to carry out a sol-gel reaction to obtain the NFPP@C@PDMS precursor.

[0122] S4. High-temperature sintering: In an argon atmosphere, the temperature is increased to 600 ℃ at 2 ℃ / min and held for 10 h to obtain Na4Fe3(PO4)2P2O7@C composite cathode material.

[0123] Electrochemical performance testing: The above-mentioned cathode material, Super P, and PVDF were mixed in a mass ratio of 80:10:10 to form a slurry, which was then coated onto carbon-coated aluminum foil and vacuum dried at 100 °C for 2 h to prepare an electrode film. The film was punched into a 1.2 mm diameter disc, using metallic sodium as the counter electrode, 1 mol / L NaPF6 (EC:DEC=1:1, v / v) as the electrolyte, and a PP / PE / PP three-layer membrane as the separator, to assemble a CR2016 coin cell. Constant current charge-discharge tests were conducted in the range of 2.0–3.6 V, with a current density of 0.1C (1C=129 mA / g).

[0124] Comparative Example 4 (discontinuous polydimethylsiloxane coverage)

[0125] This embodiment relates to an NFPP@PDMS composite cathode material, the preparation method of which includes the following steps:

[0126] S1. Preparation of precursors: NFPP precursors were synthesized using NaCl, Fe(CH3COO)2 and (NH4)H2PO4 as raw materials, with a sodium-to-phosphorus ratio of 1.003 and an iron-to-phosphorus ratio of 0.723, via a sol-gel method.

[0127] S2. Mixing of organic carbon sources: Add 8 wt% polyvinylpyrrolidone (PVP) as a carbon source, ball mill for 5 h to obtain NFPP@C.

[0128] S3, Polydimethylsiloxane coating treatment: 1.0% polydimethylsiloxane, 1.8% phenyltrimethoxysilane, and 0.15% tetraisopropyl titanate from Application Example 5 were added and reacted in an ethanol solution with stirring. Only discontinuous dotted coating layers were formed in some areas to obtain the NFPP@C@PDMS precursor.

[0129] S4. High-temperature sintering: Sintering at 580 °C for 12 h under a nitrogen atmosphere yielded Na4Fe3(PO4)2P2O7@C composite cathode material.

[0130] Electrochemical performance testing: The above-mentioned cathode material, Super P, and PVDF were mixed in a mass ratio of 80:10:10 to form a slurry, which was then coated onto carbon-coated aluminum foil and vacuum dried at 100 °C for 2 h to prepare an electrode film. The film was punched into a 1.2 mm diameter disc, using metallic sodium as the counter electrode, 1 mol / L NaPF6 (EC:DEC=1:1, v / v) as the electrolyte, and a PP / PE / PP three-layer membrane as the separator, to assemble a CR2016 coin cell. Constant current charge-discharge tests were conducted in the range of 2.0–3.6 V, with a current density of 0.1C (1C=129 mA / g).

[0131] To effectively evaluate the technical effects of the present invention, the performance of the above embodiments was evaluated, and the evaluation results are shown in Table 1:

[0132] Table 1 Performance Test Results

[0133] Example number Initial charging capacity (mAh·g⁻¹) Initial discharge capacity (mAh·g⁻¹) Initial Coulomb efficiency (%) Rechargeable after absorbing water (mAh·g⁻¹) Discharge after water absorption (mAh·g⁻¹) Initial coulombic efficiency (%) after water absorption Application Example 1 116.16 102.74 88.45% 110.73 98.59 89.04% Application Example 2 114.41 100.5 87.84% 108.33 97.18 89.71% Application Example 3 112.86 99.73 88.37% 107.36 95.05 88.53% Application Example 4 110.34 98.57 89.33% 105.97 94.17 88.86% Application Example 5 109.29 97.28 89.01% 104.37 92.27 88.41% Comparative Example 1 105.12 90.96 86.53% 96.01 83.78 87.26% Comparative Example 2 92.77 72.26 77.89% 86.65 66.5 76.75% Comparative Example 3 100.4 89.4 89.04% 91.19 80.81 88.62% Comparative Example 4 98.9 85.04 85.99% 90.86 78.2 86.07%

[0134] As shown in Table 1, in Application Example 1, the thin and dense polydimethylsiloxane coating provides excellent physical barrier properties, effectively preventing water / oxygen from directly penetrating to the NFPP surface and preventing Na⁺ / H⁺ exchange and Fe dissolution. The carbonization of 5 wt% glucose at 590°C forms a conductive carbon network, maintaining high electronic conductivity and reducing the adverse effects of the coating layer on electron transport. In contrast, in Comparative Example 1, carbon cannot effectively prevent H⁺ / O⁂ penetration; therefore, after water absorption, Na⁺ / H⁺ exchange occurs, generating NaOH / Na₂CO₃, and localized Fe oxidation and dissolution occur, resulting in significant capacity loss.

[0135] In Application Example 2, the polydimethylsiloxane protective layer is more uniform and potentially denser, thus exhibiting the strongest water-blocking effect in the water absorption test and resulting in a higher capacity retention rate. Comparing Application Example 1 and Application Example 2, it can be seen that a denser polydimethylsiloxane coating improves moisture and heat barrier properties but sacrifices some initial reversible capacity.

[0136] Compared to Comparative Example 1, Application Example 3 shows that the introduction of organic groups improves surface hydrophobicity and interfacial compatibility. A comprehensive comparison of Comparative Examples 4 and 5 demonstrates that the carbon source and silicon modifier have a significant impact on electrochemical / hygrothermal behavior. PEG is more conducive to the formation of uniform conductive carbon, and the phenyl group in PTES improves hydrophobicity; although APTES has a chemical anchoring effect, improper carbon source or reaction conditions can negatively affect conductivity.

[0137] Comparative Example 2 demonstrates that without a conductive carbon network, even with good coating, high capacity and high initial coulombic efficiency cannot be achieved. Comparative Example 3 shows that an excessively thick polydimethylsiloxane layer reduces initial reversible capacity and the relative retention rate after water absorption. This may be because although the waterproofing is good, the ionic / conductivity is impaired, leading to a decrease in overall reversibility. In Comparative Example 4, due to the discontinuous protection of polydimethylsiloxane, local exposure leads to accelerated local corrosion and dissolution, resulting in overall performance worse than other samples.

[0138] The above embodiments are merely application examples of the present invention, and their descriptions are relatively specific and detailed, but they should not be construed as limiting the scope of the present invention. It should be noted that those skilled in the art can make several modifications and improvements without departing from the concept of the present invention, and these obvious substitutions all fall within the protection scope of the present invention.

Claims

1. A method for preparing a sodium-ion battery cathode material with high water and oxygen stability, characterized in that... Includes the following steps: S1. Preparation of precursor: Iron source, sodium source and phosphorus source are mixed and then NFPP precursor is prepared; S2: Organic carbon source mixing treatment: Add organic carbon source to NFPP precursor and mix evenly to obtain NFPP@C intermediate precursor powder; S3. Polydimethylsiloxane coating deposition: After ultrasonically dispersing the NFPP@C intermediate in an ethanol solution, the hydroxyl end-capping agent, crosslinking agent, and catalyst are added and mixed evenly to deposit a polydimethylsiloxane layer on the surface of the NFPP particles in situ, thus obtaining the NFPP@PDMS precursor. S4. High-temperature sintering: After high-temperature sintering under protection, the final NFPP@PDMS composite cathode material is obtained.

2. The method for preparing the sodium-ion battery cathode material with high water and oxygen stability according to claim 1, characterized in that: In step S3, the hydroxyl end-capping agent is one or more of dihydroxy polydimethylsiloxane, diphenylsiloxane copolymer, trifluoropropylmethylsiloxane copolymer, and polydimethylsiloxane.

3. The method for preparing the sodium-ion battery cathode material with high water and oxygen stability according to claim 1, characterized in that: In step S3, the crosslinking agent is one or more of the following: tetraethyl orthosilicate, methyl orthosilicate, methyltrimethoxysilane, phenyltrimethoxysilane, polydimethylsiloxane, polymethylhydrosiloxane, and 3-aminopropyltriethoxysilane.

4. The method for preparing the sodium-ion battery cathode material with high water and oxygen stability according to claim 3, characterized in that: In step S3, the catalyst is one or more of dibutyltin dilaurate, dibutyltin diacetate, tetraisopropyl titanate, and isopropanol chloroplatinate.

5. The method for preparing the sodium-ion battery cathode material with high water and oxygen stability according to claim 4, characterized in that: In step S3, the amount of hydroxyl capping agent added is 0.5-5.0 wt%, the amount of crosslinking agent added is 0.2-3.0 wt%, and the amount of catalyst added is 0.01-0.5 wt%.

6. The method for preparing the sodium-ion battery cathode material with high water and oxygen stability according to claim 1, characterized in that: In step S4, the protective atmosphere is nitrogen or argon, the sintering temperature is 580-620℃, and the heating rate is 1-5℃ / min. -1 .

7. The method for preparing the sodium-ion battery cathode material with high water and oxygen stability according to claim 1, characterized in that: In step S2, the organic carbon source is one or more of glucose, sucrose, fructose, citric acid, oxalic acid, acetic acid, tartaric acid, polyethylene glycol, and polyvinylpyrrolidone, and the amount of the organic carbon source added is 2.0-5.0% of the mass of the NFPP precursor.

8. The method for preparing the sodium-ion battery cathode material with high water and oxygen stability according to claim 1, characterized in that: In step S1, the molar ratio of sodium to phosphorus is 1.002-1.014, and the molar ratio of iron to phosphorus is 0.710-0.

725.

9. The method for preparing the sodium-ion battery cathode material with high water and oxygen stability according to claim 1, characterized in that: In step S1, the sodium source is one or more of Na2CO3, NaHCO3, NaOH, NaNO3, NaH2PO4, Na2HPO4, Na4P2O7, and NaCl; the iron source is one or more of FeC2O4·2H2O, Fe(NO3)3·9H2O, FeSO4·7H2O, FePO4, FeO, Fe2O3, Fe3O4, and Fe(CH3COO)2; and the phosphorus source is one or more of H3PO4, (NH4)H2PO4, NaH2PO4, Na2HPO4, (NH4)2HPO4, Na4P2O7, K4P2O7, and (NaPO3)6.

10. The method for preparing the sodium-ion battery cathode material with high water and oxygen stability according to claim 1, characterized in that: In step S1, the NFPP precursor is prepared by sol-gel method or spray drying method; in step S2, the uniform mixing method is ball milling or stirring.