Surface-modified sodium ferric sulfate positive electrode material and preparation method thereof

Abstract

The application relates to a preparation method of a surface-modified sodium ferric sulfate positive electrode material with good processing performance and hydrophobic performance, which comprises the following steps: (1) providing a carbon-composited sodium ferric sulfate precursor; (2) depositing a hydrophobic group onto the material surface by using a low-temperature plasma deposition technology to obtain a sodium ferric sulfate positive electrode material with a core-shell structure. The low-temperature plasma deposition technology is used to construct a hydrophobic thin shell layer on the surface of the sodium ferric sulfate precursor material, so that the air stability, processing and thermal stability of the material are greatly improved, meanwhile, the stability of the positive electrode structure is enhanced, the pulverization of the positive electrode material particles in the cycle process is prevented, and meanwhile, the inner core is a carbon-composited conductive active network structure, which is beneficial to the transmission of sodium ions.

Description

Technical Field

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

[0002] Lithium-ion battery technology has undergone long-term development and possesses excellent electrochemical performance, leading to its widespread application in various aspects of life. However, lithium metal resources on Earth are not abundant. Furthermore, the demand for renewable energy and grid-balanced energy storage is enormous. Sodium-ion batteries (SIBs), as a low-cost alternative to lithium-ion batteries, have regained attention due to abundant sodium resources and chemical properties similar to lithium. Developing cathode materials using elements abundant on Earth (such as iron) could further reduce costs.

[0003] Currently, sodium-ion cathode materials include layered oxides, Prussian blue and its analogues, and polyanionic materials, each with its own advantages and disadvantages. For example, layered oxides often undergo multiple phase transitions during the redox process of sodium insertion / extraction, leading to structural collapse and affecting the material's cycle stability. Prussian blue materials, for instance, contain a large amount of lattice water during synthesis, which decomposes at high potentials to generate a large amount of gas, causing battery swelling and failure. Compared to the first two types of materials, sodium ferric sulfate, a polyanionic material, has a promising application prospect in sodium-ion cathodes due to its stable 3D framework structure and excellent electrochemical performance. However, one of the key problems with sodium ferric sulfate is its sensitivity to air. Due to its interaction with moisture and oxygen in the air, sodium ferric sulfate exhibits surface alkali residue, and some ferrous ions are oxidized, increasing the difficulty of slurry preparation and coating, and deteriorating specific capacity and cycle performance.

[0004] This invention utilizes low-temperature plasma deposition technology to construct a water-conducting thin shell layer on the surface of sodium ferric sulfate precursor materials, forming a unique core-shell structure that significantly improves the material's hydrophobicity, processability, and thermal stability. It also enhances the stability of the cathode structure, preventing pulverization of cathode material particles during cycling. Simultaneously, the core is a carbon composite conductive active network structure, facilitating sodium ion transport. Superhydrophobic films can be obtained without the use of any organic surface modifiers. This method offers advantages such as simple synthesis, environmental friendliness, and high film adhesion, making it suitable for industrial production. Summary of the Invention

[0005] The purpose of this invention is to construct a water-conducting thin shell layer on the surface of sodium ferric sulfate precursor material using low-temperature plasma deposition technology, forming a unique core-shell structure that greatly improves the hydrophobicity, processing, and thermal stability of the material.

[0006] To achieve the above objectives, the present invention provides the following technical solution:

[0007] A method for preparing a surface-modified sodium ferric sulfate cathode material includes the following steps:

[0008] a. Provides a carbon-complexed sodium ferric sulfate precursor;

[0009] b. Constructing a plasma-deposited hydrophobic group film: A mask is fixed on the surface of a carbon-composite sodium ferric sulfate precursor substrate. A plasma source is prepared using a high-frequency plasma generator to generate the working gas providing the hydrophobic groups. The parameters for low-temperature plasma deposition are optimized. The hydrophobic groups are deposited onto the precursor surface using a mask-spray deposition method, and then the mask is removed. Hydrophobic groups (-C) n H 2n+1 (e.g., -CH=CH2 and -C6H5) are deposited onto the surface of the material to obtain a core-shell structured sodium iron sulfate cathode material.

[0010] In step a: first, conductive carbon and zirconium balls are premixed at low speed, then an iron source, a sulfur source, and a sodium source are added for dry ball milling, followed by high-temperature calcination under nitrogen protection, and finally crushing and sieving to obtain a carbon-composite sodium iron sulfate precursor. Premixing the conductive carbon nanotubes and zirconium balls at low speed in advance can effectively alleviate the agglomeration of subsequent materials.

[0011] In step a: the precursor chemical formula Na x Fe y SO4 / C, where 0.75≤x≤0.90, 0.55≤y≤0.65 and 1.2≤x / y≤2.

[0012] In step a: the conductive carbon source material can be one or a mixture of two of the following: one-dimensional graphite particles, two-dimensional graphene, acetylene black, and three-dimensional single-walled or multi-walled carbon nanotubes.

[0013] In step a: the conductive carbon accounts for 1-5% of the total mass of the precursor.

[0014] In step a: the calcination temperature is 330-380℃, the humidity inside the furnace is controlled to be ≤3%, nitrogen atmosphere is used for calcination, and the oxygen content inside the furnace is less than 5ppm.

[0015] The crushing and sieving conditions in step a are as follows: inlet pressure: 0.1-3MPa; airflow crushing frequency: 50-100Hz; grading frequency: 10-150Hz; sieve mesh size: 400-500 mesh.

[0016] In step b: the mesh count of the mask is 50-800 mesh, with 60-100 mesh being optimal;

[0017] In step b: the working gas is gaseous alkanes such as methane, ethane, and propane; gaseous olefins such as ethylene and propylene; gaseous cycloalkanes such as cyclopropane and cyclohexane; and gaseous alkynes such as acetylene and propyne.

[0018] In step b, the deposition temperature of the substrate material is 100-300℃, with the optimum being 200-250℃.

[0019] The parameters for step b, low-temperature plasma deposition, are: RF power 100-200W, RF frequency 14MHz, substrate temperature 200-250℃, cavity pressure 50-100Pa, gas flow rate 20-60sccm, and deposition time 10-30min.

[0020] The present invention also provides a surface-modified sodium ferric sulfate cathode material prepared by the above preparation method.

[0021] The present invention also provides the application of the above-mentioned cathode material in sodium-ion batteries.

[0022] The specific innovative points of this invention are reflected in the following aspects:

[0023] (1) In this invention, when raw materials and conductive carbon materials are bulk composited, a good conductive network is formed in the core, which is conducive to the diffusion of sodium ions and thus improves the electrochemical performance of the material.

[0024] (2) The present invention uses a mask spray deposition method to deposit hydrophobic groups onto the surface of the cathode material. Compared with the direct gas flow deposition method, it has better deposition uniformity and avoids the aggregation of local hydrophobic groups.

[0025] (3) Compared with CVD process, low temperature plasma deposition has much lower temperature requirements for substrate, which is suitable for surface hydrophobic modification of sodium iron sulfate cathode material (which is easily decomposed above 400℃); compared with other organic hydrophobic surface modifier processes, low temperature plasma deposition avoids the capacity reduction problem caused by material contact with water.

[0026] (4) This invention constructs a hydrophobic thin shell layer on the surface of sodium ferric sulfate precursor material using low-temperature plasma deposition technology, forming a unique core-shell structure. The thin hydrophobic film layer of the outer shell can be obtained without the use of any organic surface modifiers; at the same time, compared with the film layer adsorbed by intermolecular forces, the hydrophobic groups deposited by this method through chemical bonds have higher adhesion and better air stability. Attached Figure Description

[0027] Figure 1 This is a schematic diagram of a surface-modified sodium ferric sulfate cathode material.

[0028] Figure 2 for Na0.89 Fe 0.56 SEM image of SO4 / 2% CNTs@film cathode material. Detailed Implementation

[0029] To make the technical solutions and advantages of the present invention clearer, the present invention will be further described in detail below with reference to embodiments. It should be understood that the specific embodiments described herein are only for explaining the present invention and are not intended to limit the present invention.

[0030] Example 1

[0031] The first step involves providing a carbon-composite sodium ferric sulfate precursor: 1.75 kg of FeSO4, 1.31 kg of Na2SO4, and 0.062 kg of carbon nanotubes (CNTs) are weighed; the precursors are uniformly compounded using a dry ball milling method, with high-purity nitrogen as the protective gas, then compressed into tablets and calcined at 350°C to obtain the sodium ferric sulfate precursor. The CNTs constitute 2% wt of the precursor. The precursor is then pulverized and sieved to obtain a uniform particle size distribution of 3.0 μm.

[0032] The second step involves constructing a plasma-deposited hydrophobic group film:

[0033] (1) Take a uniformly sized sodium ferric sulfate precursor substrate and fix a mask with a certain mesh size on the surface. The mesh size of the mask is 60 mesh.

[0034] (2) Place the glass substrate from step (1) into the low-temperature plasma deposition chamber, use methane as the working gas, set the low-temperature plasma deposition parameters, and deposit the substrate.

[0035] (3) The low-temperature plasma deposition parameters are: RF power 250W, RF frequency 14.0MHz, substrate temperature 200℃, cavity pressure 60Pa, flow rate of methane gas with purity of 99.999% 40sccm, and deposition time 10min.

[0036] After film deposition using the above process, the resulting film thickness is 65 nm, and the obtained cathode material is Na. 0.89 Fe 0.56 SO4 / 2% CNTs@film, its structural diagram is as follows Figure 1 As shown, the SEM image is as follows: Figure 2 As shown.

[0037] Example 2

[0038] The first step provides a carbon-composite sodium ferric sulfate precursor: the process parameters are the same as in Example 1;

[0039] The second step is to construct a plasma-deposited hydrophobic group film: the deposition time is modified to 20 min, and the other parameters are the same as those in Example 1.

[0040] After film deposition using the above process, the resulting film thickness is 75 nm, and the resulting cathode material is Na. 0.89 Fe 0.56 SO4 / 2% CNTs@film.

[0041] Example 3

[0042] The first step provides a carbon-composite sodium ferric sulfate precursor: the process parameters are the same as in Example 1;

[0043] The second step is to construct a plasma-deposited hydrophobic group film: the flow rate of the working gas methane is modified to 50 sccm, the deposition time is 10 min, and the other parameters are the same as those in Example 1.

[0044] After film deposition using the above process, the resulting film thickness is 72 nm, and the obtained cathode material is Na. 0.89 Fe 0.56 SO4 / 2% CNTs@film.

[0045] Example 4

[0046] The first step provides a carbon-composite sodium ferric sulfate precursor: the process parameters are the same as in Example 1;

[0047] The second step is to construct a plasma-deposited hydrophobic group film: the flow rate of the working gas methane is modified to 60 sccm, the deposition time is 10 min, and the other parameters are the same as those in Example 1.

[0048] After film deposition using the above process, the resulting film thickness is 80 nm, and the resulting cathode material is Na. 0.89 Fe 0.56 SO4 / 2% CNTs@film.

[0049] Example 5

[0050] The first step involves providing a carbon-composite sodium ferric sulfate precursor: 1.75 kg of FeSO4, 1.31 kg of Na2SO4, and 0.062 kg of conductive carbon black (Super P) are weighed; the precursor is uniformly compounded using a dry ball milling method, with high-purity nitrogen as the protective gas, then pressed into tablets and calcined at 350°C to obtain the sodium ferric sulfate precursor. The conductive carbon black comprises 2% wt. The precursor is then pulverized and sieved to obtain a uniform sodium ferric sulfate precursor with a particle size distribution of 3.0 μm.

[0051] The second step involves constructing a plasma-deposited hydrophobic group film: the parameters are the same as those in Example 1.

[0052] After film deposition using the above process, the resulting film thickness is 65 nm, and the obtained cathode material is Na. 0.89 Fe 0.56 SO4 / 2% SuperP@film.

[0053] Comparative Example 1

[0054] Preparation of carbon-composite sodium ferric sulfate precursor: 1.75 kg of FeSO4, 1.31 kg of Na2SO4, and 0.062 kg of conductive carbon nanotubes (CNTs) were weighed; the precursors were uniformly compounded by dry ball milling, using high-purity nitrogen as a protective gas, pressed into tablets, and calcined at 350 °C to obtain the sodium ferric sulfate precursor. The proportion of conductive carbon nanotubes (CNTs) was 2% wt. The precursor was then pulverized and sieved to obtain a uniform particle size distribution of 3.0 μm. The cathode material obtained by the above process is Na... 0.89 Fe 0.56 SO4 / 2% CNTs.

[0055] Comparative Example 2

[0056] The first step provides a carbon-composite sodium ferric sulfate precursor: the process parameters are the same as in Example 1;

[0057] The second step is to construct a plasma-deposited hydrophobic group film: the working gas is changed to ethylene with a purity of 99.999%, and the other parameters are the same as the process parameters in Example 1;

[0058] After film deposition using the above process, the resulting film thickness is 64 nm, and the obtained cathode material is Na. 0.89 Fe 0.56 SO4 / 2% CNTs@film.

[0059] Comparative Example 3

[0060] The first step provides a carbon-composite sodium ferric sulfate precursor: the process parameters are the same as in Example 1;

[0061] The second step is to construct a plasma-deposited hydrophobic group film: the deposition time is modified to 5 min, and the other parameters are the same as those in Example 1.

[0062] After film deposition using the above process, the resulting film thickness is 32 nm, and the obtained cathode material is Na. 0.89 Fe 0.56 SO4 / 2% CNTs@film.

[0063] Comparative Example 4

[0064] The first step provides a carbon-composite sodium ferric sulfate precursor: the process parameters are the same as in Example 1;

[0065] The second step is to construct a plasma-deposited hydrophobic group film: the flow rate of the working gas methane is modified to 80 sccm, the deposition time is 20 min, and the other parameters are the same as those in Example 1.

[0066] After film deposition using the above process, the resulting film thickness is 150 nm, and the resulting cathode material is Na. 0.89 Fe 0.56 SO4 / 2% CNTs@film.

[0067] Table 1 shows the electrochemical performance results of the surface-modified sodium ferric sulfate cathode material in the specific embodiments.

[0068]

[0069] As shown in Table 1, the surface-modified sodium ferric sulfate cathode material prepared by this invention exhibits good processing performance and hydrophobic properties. The optimal CNT content in the ball-milled composite is 2%. Comparing Examples 1 and 2, the film thickness increases with increasing plasma deposition time, resulting in improved rate performance but reduced specific capacity. Comparing Examples 1, 3, and 4, the film thickness increases with increasing working gas flow rate during plasma deposition, improving capacity retention after 500 cycles but reducing reversible capacity. Furthermore, the rate performance of the material also increases, indicating that a suitable hydrophobic surface film can provide a sufficient transport channel for the core active material to contact the electrolyte, thereby improving the rate performance. Examples 1 and 5 show that replacing the composite conductive carbon material with acetylene black does not significantly reduce the electrochemical performance of the material, representing a promising direction for cost reduction. Comparing Example 1 and Comparative Example 1 reveals that without the introduction of a hydrophobic film layer, the electrochemical performance of the materials decreases, and the capacity ratio performance of the materials without the hydrophobic film layer also decreases, indicating that the hydrophobic film layer can effectively reduce water absorption and thus improve capacity. Comparing Example 1 and Comparative Example 2 shows that when the working gas is acetylene, the first-cycle coulombic efficiency and reversible capacity of the material decrease, but the long-cycle stability is slightly improved. Comparing Example 1, Comparative Example 3, and Comparative Example 4 shows that the thickness of the hydrophobic film layer has a significant impact on the physicochemical properties of the material. When the film layer thickness is less than 50 nm or greater than 150 nm, the electrochemical performance of the material decreases significantly, indicating that the hydrophobic film layer can significantly affect the insertion / extraction and diffusion of sodium ions in the material system.

[0070] The above are merely preferred embodiments of the present invention and are not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for preparing a surface-modified sodium ferric sulfate cathode material, characterized in that, Includes the following steps: a. Provides a carbon-complexed sodium ferric sulfate precursor; b. Constructing a plasma-deposited hydrophobic group film: A mask is fixed on the surface of a carbon-composite sodium iron sulfate precursor substrate. A plasma source is prepared using a high-frequency plasma generator to generate the working gas that provides the hydrophobic groups. The PECVD deposition parameters are optimized, and the hydrophobic groups are deposited onto the precursor surface through a mask spray deposition method. After removing the mask, a core-shell structured surface-modified sodium iron sulfate cathode material is obtained. The working gas is a gaseous alkane, gaseous olefin, or gaseous cycloalkanes. The PECVD deposition parameters are: RF power 100–200 W, RF frequency 14 MHz, substrate temperature 200–250 °C, cavity pressure 50–100 Pa, gas flow rate 20–60 sccm, and deposition time 10–30 min.

2. The preparation method according to claim 1, characterized in that, In step a: first, the conductive carbon source and zirconium balls are premixed at low speed, then iron source, sulfur source and sodium source are added for dry ball milling, then calcined at high temperature under nitrogen protection, and finally crushed and sieved to obtain carbon composite sodium iron sulfate precursor.

3. The preparation method according to claim 2, characterized in that, In step a: the precursor ratio is based on the chemical formula Na x Fe y SO4 / C, wherein 0.75≤x≤0.90, 0.55≤y≤0.65 and 1.2≤x / y≤2, and the carbon content in the total mass of the precursor is 1-5%.

4. The preparation method according to claim 2, characterized in that, In step a: the conductive carbon source material is one or a mixture of two of the following: one-dimensional graphite particles, two-dimensional graphene, acetylene black, and three-dimensional single-walled or multi-walled carbon nanotubes.

5. The preparation method according to claim 2, characterized in that, In step a: the calcination temperature is 330-380 ℃, the humidity inside the furnace is controlled to be ≤3%, calcination is carried out in a nitrogen atmosphere, and the oxygen content inside the furnace is less than 5ppm; the crushing and sieving conditions are as follows: air inlet pressure: 0.1-3MPa; airflow crushing frequency: 50-100Hz; grading frequency: 10-150Hz; sieve mesh: 400-500 mesh.

6. The preparation method according to claim 1, characterized in that, In step b: the mesh size of the mask is 50-800 mesh.

7. A surface-modified sodium ferric sulfate cathode material prepared by the preparation method according to any one of claims 1-6.

8. The application of a surface-modified sodium iron sulfate cathode material as described in claim 7 in a sodium-ion battery.

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