Preparation method of high-rate performance sodium sulfate iron positive electrode material, positive electrode material, electrode sheet and sodium ion battery

Abstract

The application relates to the field of sodium ion batteries, and discloses a preparation method of a high-rate performance sodium ferrosulfate positive electrode material, the positive electrode material, an electrode sheet and a sodium ion battery. The iron source, the sodium source and the carbon source are subjected to wet ball milling under the protection of an inert atmosphere, and after drying and grinding, a mixed precursor powder is obtained; the precursor powder is pressed into a dense precursor sheet; the precursor sheet is subjected to joule heat calcination under the protection of an inert atmosphere, and a sodium ferrosulfate positive electrode material with oxygen vacancies is obtained. The method significantly improves the electronic conductivity of the material, and further greatly improves the rate performance of the material. In addition, the preparation process has the advantages of extremely fast heating rate, low energy consumption and simple process, and is suitable for large-scale industrial production. By adjusting the impact time, impact temperature and holding time, a high-purity sodium ferrosulfate phase is synthesized, and the sodium ferrosulfate phase has good sodium storage specific capacity and excellent rate performance.

Description

Technical Field

[0001] This invention belongs to the field of sodium-ion batteries and relates to a method for preparing a high-rate performance sodium iron sulfate cathode material, the cathode material, the electrode sheet, and the sodium-ion battery. Background Technology

[0002] Sodium-ion batteries have great potential for large-scale energy storage applications due to the abundance of sodium resources, low cost, good safety, and electrochemical energy storage mechanism similar to lithium-ion batteries. The cathode material, as a core component determining battery energy density, cycle life, and safety, has attracted significant industry attention regarding its technological approach. Currently, the three mainstream cathode material systems each have their own strengths: layered oxides have a leading advantage in the power field due to their higher energy density, but face capacity decay issues caused by structural phase transitions during cycling; Prussian blue analogues are suitable for specific niche applications due to their low cost and high power characteristics, but challenges in controlling crystal water and purity restrict long-term reliability; while polyanionic compounds, with their unique structural advantages, possess both excellent structural stability and an ultra-high voltage platform, becoming a core potential direction for overcoming the performance bottlenecks in the industrialization of sodium-ion batteries. Among them, sodium iron sulfate has the highest operating potential (approximately 3.8V), and its raw material and production costs (400 yuan / ton) are far lower than those of sodium iron pyrophosphate and sodium iron phosphate, but it still suffers from poor electronic conductivity, poor air stability, and difficulty in synthesizing a pure phase.

[0003] While existing technologies attempt to improve conductivity or cycling stability through methods such as carbon coating (e.g., CN117613248A), heteroelement doping (e.g., CN120208298A), or the preparation of single-crystal materials (e.g., CN119542399A), these methods are all external modifications or structural optimizations, and therefore fail to fundamentally solve the problem of insufficient electronic and ionic conductivity within the bulk material. In particular, the capacity retention of existing methods at ultra-high rates (e.g., >20C) remains unsatisfactory.

[0004] Therefore, there is an urgent need for a material modification strategy that is simple and efficient in preparation and can fundamentally improve the electronic conductivity and ion diffusion capacity of sodium ferric sulfate simultaneously, so as to break through its high-rate performance bottleneck. Summary of the Invention

[0005] To address the aforementioned problems, the solution of this invention is to break away from the traditional calcination method of tubular furnaces and use Joule heating to calcine sodium ferric sulfate material on the basis of carbon coating. This eliminates the heat transfer process and simplifies the process, thereby improving both the ionic and electronic conductivity of sodium ferric sulfate and solving the problem that sodium ferric sulfate cannot achieve high-rate calcination in the prior art.

[0006] Based on this, the present invention proposes a method for preparing high-rate performance sodium iron sulfate cathode material, cathode material, electrode sheet, and sodium-ion battery.

[0007] The technical solution of this invention is: In a first aspect, the present invention provides a method for preparing a high-rate performance sodium ferric sulfate cathode material, which is synthesized by Joule heating calcination and includes the following steps: (1) The iron source, sodium source and carbon source are wet ball milled under an inert atmosphere, and after drying and grinding, a uniformly mixed precursor powder is obtained; (2) The precursor powder is pressed into a compacted precursor sheet. (3) The precursor sheet is subjected to Joule heating under an inert atmosphere, wherein the calcination conditions are: impact heating time 3-7s, calcination temperature 500-700℃, and holding time ≤1s, to obtain sodium iron sulfate cathode material with oxygen vacancies.

[0008] Furthermore, the Joule heating impact heating time is 5-6 s, the calcination temperature is 550-700℃, and the heating rate is 100-200℃ / s.

[0009] Further, in step (1), the iron source is at least one of ferrous sulfate heptahydrate, ferrous sulfate monohydrate, or anhydrous ferrous sulfate; the sodium source is at least one of sodium sulfate or anhydrous sodium sulfate; and the carbon source is at least one of carbon nanotubes, acetylene black, or Ketjen black. The Na:Fe molar ratio of the sodium source and the iron source is 1.2-1.6:1. The material prepared by mixing the sodium and iron sources with this molar ratio has excellent sodium storage electrochemical performance and can provide a high specific capacity.

[0010] The carbon source accounts for 5%-10% of the total mass of the iron source, sodium source, and carbon source; Before use, the carbon source needs to be heat-treated in a vacuum oven at 80°C for 8-12 hours to remove surface adsorbed water and / or crystal water, thus obtaining a dry carbon source.

[0011] Furthermore, in step (1), the ball milling speed is 300-400 r / min, the ball milling time is 10-14 h, the vacuum drying temperature is 60-80 ℃, and the time is 3-8 h.

[0012] Furthermore, in step (1), the grinding media used in the ball mill are all zirconia balls, with a ratio of 1:2 between zirconia balls with a diameter of 5 mm and zirconia balls with a diameter of 3 mm, and the ball-to-material ratio is set to 10:1-20:1.

[0013] Furthermore, in step (2), the tableting process involves placing the precursor powder in a tablet press and pressing it at a pressure of 5-10 MPa for 3-5 seconds.

[0014] Furthermore, the inert atmosphere described in steps (1) and (3) is nitrogen or argon.

[0015] Furthermore, the sodium source is dehydrated before use: the sodium source and carbon source are heat-treated in a vacuum oven at 180-220℃ for 5-8 hours to remove surface adsorbed water and / or crystal water.

[0016] Furthermore, the carbon source is placed in a vacuum oven at 70-90℃ for 7-10 hours for heat treatment.

[0017] Furthermore, the Joule heating process described in step (3) specifically includes: S1. Cut the carbon fiber cloth to 6cm. Place a 2cm sample on the sintering table, place the precursor sheet in the middle of the carbon cloth, align the infrared thermometer with the sample, clamp the terminals at both ends with alligator clips, and close the vacuum chamber door to prepare for calcination. S2. Under inert gas protection, the sample is heated to 500-700℃ with a shock time of 3-7s, held for 1s, and then the program ends, the temperature drops rapidly, and the sample cools to room temperature.

[0018] The Joule heating impact time is controlled at 3-7 seconds, and the holding time is controlled at 1 second. If the time is too long, Fe2O3 or Na6Fe(SO4)4 impurities will appear, reducing the phase purity. If the time is too short, the reaction will be insufficient, and the material synthesis speed will be too fast, making it difficult to form the phase.

[0019] The synthesis temperature is controlled between 500-700℃. Samples synthesized below 500℃ have poor crystallinity, resulting in low electrical conductivity and poor electrochemical performance. Samples synthesized above 700℃ decompose sulfate and produce impurity phases.

[0020] Secondly, the present invention provides a sodium iron sulfate cathode material prepared by the above-mentioned preparation method, wherein the material has oxygen vacancies, and the oxygen vacancy content is characterized by the signal intensity of the electron paramagnetic resonance (EPR) spectrum at g=2.003.

[0021] Furthermore, the specific capacity of the sodium ferric sulfate cathode material at a 50C rate is not less than 75 mAh / g.

[0022] Furthermore, the sodium ion diffusion coefficient of the sodium ferric sulfate cathode material is 10. -10 -10 -14 cm 2 ·s -1 .

[0023] Thirdly, the present invention provides an electrode sheet comprising the above-mentioned sodium ferric sulfate positive electrode material.

[0024] Fourthly, the present invention provides a sodium-ion battery, comprising a positive electrode, a negative electrode, a separator, and an electrolyte, wherein the positive electrode uses the aforementioned electrode sheet.

[0025] Furthermore, the negative electrode is a sodium metal sheet.

[0026] Furthermore, the preparation method of the positive electrode is as follows: Sodium ferric sulfate positive electrode material Na... 2.6 Fe 1.7 (SO4)3, super P and polyvinylidene fluoride (PVDF) are ground and mixed in a mass ratio of 8:1:1. N-methylpyrrolidone (NMP) is added and mixed into a homogenate. The homogenate is then uniformly coated onto an aluminum current collector under a drying environment and dried in a vacuum oven at 90-110℃ for 10-14 hours.

[0027] Furthermore, the solute in the electrolyte was 1 M sodium perchlorate (NaClO4), and the solvent was prepared by adding 5% by volume of fluoroethylene carbonate (FEC) to a mixture of ethylene carbonate (EC) and propylene carbonate (PC) in a volume ratio of 1:1. The test temperature was room temperature.

[0028] The beneficial effects of this invention are as follows: (1) The sodium iron sulfate cathode material prepared by Joule heating in this invention has oxygen vacancy formed by the escape of lattice oxygen in the material. The released electrons can easily jump to the conduction band to form free charge carriers and directly participate in charge transport, thereby improving electronic conductivity.

[0029] (2) The partial escape of oxygen atoms in the original ferrite hexahedron reduces the splitting energy of the t2g and eg orbitals of the d orbitals of Fe, which is conducive to the transition of electrons between different energy levels, reduces the degree of localization, and further improves the conductivity of the material.

[0030] (3) Local lattice phenomena disrupt the integrity of the ideal lattice. Ions no longer diffuse continuously along fixed, narrow lattice gaps. The O-Na-O interlayer spacing widens, and the sodium ion diffusion channels expand. + The insertion / extraction barrier and impedance are reduced, which enhances the migration rate of sodium ions inside the material and improves cycling performance at high rates. (4) Carbon coating forms a continuous conductive path on the material surface, significantly improving the electronic conductivity of the material. This accelerates the charge transfer rate and improves the overall reaction kinetics; (5) By combining ball milling with Joule heating, there is no energy transfer or consumption of heat. When the current passes through the material, the material’s internal resistance hinders the current from generating heat, resulting in higher energy utilization efficiency. It has significant advantages in terms of production and manufacturing costs, high-rate performance and long-cycle stability. Attached Figure Description

[0031] Figure 1 X-ray powder diffraction (XRD) patterns of the cathode materials prepared in Examples 1-8 and Comparative Examples 1-2; Figure 2The X-ray powder diffraction (XRD) patterns are shown for the cathode materials prepared in Comparative Examples 3-6. Figure 3 The following are the Fourier Transform Infrared (FTIR) spectra of the cathode materials prepared in Examples 1-3 and Comparative Example 1; Figure 4 The images are scanning electron microscope (SEM) images of the cathode materials prepared in Examples 1-4, where a is Example 1, b is Example 2, c is Example 3, and d is Example 4. Figure 5 The images are scanning electron microscope (SEM) images of the cathode materials prepared in Comparative Examples 1 and 2, where a is Comparative Example 1 and b is Comparative Example 2. Figure 6 The electron paramagnetic resonance (EPR) images are of the cathode materials prepared in Example 1 and Comparative Example 1. Figure 7 The diagram shows the apparatus used during calcination in Examples 1-8 and Comparative Examples 3-6. Figure 8 The table shows the rate performance of sodium-ion batteries prepared with cathode materials from Examples 1-4 and Comparative Example 1 at current densities of 0.1C, 0.2C, 0.5C, 1C, 2C, 5C, 10C, 20C, 30C, and 50C, and the performance after 200 charge-discharge cycles at a current density of 1C. Figure 9 The galvanostatic charge-discharge curves (GCD) of the sodium-ion battery in the first two cycles at a current density of 0.5C, where the left side corresponds to Example 1 and Comparative Example 1; and the right side corresponds to Examples 1-8 and Comparative Examples 1-2. Figure 10 The galvanostatic intermittent titration (GITT) curves are for testing sodium-ion batteries prepared with cathode materials from Example 1 and Comparative Example 1. Detailed Implementation

[0032] The present invention will be further described in detail below with reference to specific embodiments and accompanying drawings. This detailed description is a more detailed description of certain aspects, features and embodiments of the present invention, and should not be construed as limiting the scope of protection of the present invention.

[0033] Example 1 A method for preparing a high-rate performance sodium ferric sulfate cathode material includes the following steps: (1) Dehydration treatment of anhydrous sodium sulfate and Ketjen black: anhydrous sodium sulfate was heat-treated at 200°C for 6 hours in a vacuum oven, and Ketjen black was heat-treated at 80°C for 8 hours in a vacuum oven.

[0034] Weigh out 2.377g of anhydrous sodium sulfate and 3.718g of ferrous sulfate monohydrate (FeSO4). H2O), and 5% of Ketjenblack (based on the total mass of ferrous sulfate monohydrate, anhydrous sodium sulfate, and Ketjenblack) was added to a 70 mL ball mill jar. Zirconia balls with a diameter of 5 mm and a diameter of 3 mm were added in a ratio of 1:2, with a mass ratio of 10:1 between the zirconium balls and the mixture. 12 mL of acetone was injected, and the ball mill jar was sealed under an argon atmosphere. The ball milling time was 12 h at a speed of 400 r / min. After the ball milling was completed, the mixture was dried in a vacuum oven at a temperature of 60 °C for 6 h. The precursor powder was obtained after drying.

[0035] (2) The obtained precursor powder was ground and sieved through a 100-mesh sieve to collect black powder. 0.5g of black powder was weighed and placed in a tablet press and compacted for 3s under a pressure of 10MPa to obtain a precursor tablet with a thickness of about 2mm.

[0036] (3) The obtained precursor sheet was transferred to a Joule heating apparatus for Joule calcination. Under an argon atmosphere, the precursor sheet was placed in the middle of carbon cloth, and an infrared thermometer was aimed at the sample. The sample was heated to the calcination temperature of 650℃ with a shock heating time of 5s and a holding time of 1s. The heating rate was 100℃ / s. After the sintering process, the sample was rapidly cooled to room temperature to obtain sodium iron sulfate cathode material Na. 2.6 Fe 1.7 (SO4)3, denoted as NFS-J-1.

[0037] Example 2 Unlike Example 1, the Joule heating temperature was 550°C, but otherwise the same as Example 1, and it is designated as NFS-J-2.

[0038] Example 3 Unlike Example 1, the Joule heating temperature was 600°C, but otherwise the same as Example 1, and it is designated as NFS-J-3.

[0039] Example 4 Unlike Example 1, the Joule heating temperature was 700°C, but otherwise the same as in Example 1, and it is designated as NFS-J-4.

[0040] Example 5 Unlike Example 1, 2.049 g of anhydrous sodium sulfate and 4.084 g of ferrous sulfate monohydrate (FeSO4) were weighed out. The sodium ferric sulfate cathode material Na6Fe5(SO4)8 was obtained by calcining H2O at a Joule temperature of 600℃, and the rest was the same as in Example 1, denoted as NFS-J-5.

[0041] Example 6 Unlike Example 1, 2.049 g of anhydrous sodium sulfate and 4.084 g of ferrous sulfate monohydrate (FeSO4) were weighed out. H2O) was used to obtain sodium ferric sulfate cathode material Na6Fe5(SO4)8, and the rest was the same as in Example 1, denoted as NFS-J-6.

[0042] Example 7 The difference from Example 1 is that Ketjen Black is replaced with acetylene black, otherwise it is the same as Example 1, and it is denoted as NFS-J-7.

[0043] Example 8 The difference from Example 1 is that Ketjen Black is replaced with carbon nanotubes, otherwise it is the same as Example 1, and it is denoted as NFS-J-8.

[0044] Comparative Example 1 A method for preparing a high-rate performance sodium ferric sulfate cathode material includes the following steps: (1) Dehydration treatment of anhydrous sodium sulfate and Ketjen black: anhydrous sodium sulfate was heat-treated at 200°C for 6 hours in a vacuum oven, and Ketjen black was heat-treated at 80°C for 8 hours in a vacuum oven.

[0045] Weigh out 2.377g of anhydrous sodium sulfate and 3.718g of ferrous sulfate monohydrate (FeSO4). H2O was added, along with 5% Ketjenblack (based on the total mass of ferrous sulfate monohydrate, anhydrous sodium sulfate, and Ketjenblack) in a 70 mL ball mill jar. Zirconia balls (5 mm diameter and 3 mm diameter) were added in a 1:2 ratio (zirconia balls to mixture mass ratio 10:1). 12 mL of acetone was injected, and the ball mill jar was sealed under an argon atmosphere. The ball milling time was 12 h at 400 r / min. After milling, the mixture was dried in a vacuum oven at 60 °C for 6 h, yielding sodium ferric sulfate precursor powder.

[0046] (2) The obtained precursor powder was ground and sieved through a 100-mesh sieve, and black powder was collected. 0.5g of black powder was weighed and placed in a tablet press and compacted under a pressure of 10MPa for 1min to obtain a precursor tablet with a thickness of about 2mm. (3) The obtained precursor sheet was placed in a tube furnace with argon as the carrier gas, heated to 400℃ at a rate of 2℃ / min and held for 12h, and then cooled to room temperature to obtain sodium iron sulfate cathode material Na. 2.6 Fe 1.7 (SO4)3 is NFS-G-9.

[0047] Comparative Example 2 Unlike Comparative Example 1, 2.049 g of anhydrous sodium sulfate and 4.08 g of ferrous sulfate monohydrate (FeSO4) were weighed out. H2O was used to obtain sodium ferric sulfate cathode material product Na6Fe5(SO4)8, and the rest were the same as in comparative example 1, denoted as NFS-G-10.

[0048] Comparative Example 3 Unlike Example 1, the Joule heating temperature is 450°C, otherwise it is the same as Example 1, and is referred to as NFS-J-11.

[0049] Comparative Example 4 Unlike Example 1, the Joule heating temperature is 800°C, but otherwise it is the same as Example 1, and is denoted as NFS-J-12.

[0050] Comparative Example 5 Unlike Example 1, the impact heating time is 1 second, otherwise it is the same as Example 1, and is denoted as NFS-J-13.

[0051] Comparative Example 6 Unlike Example 1, the impact heating time is 15s, otherwise it is the same as Example 1, and it is denoted as NFS-J-14.

[0052] Comparative Example 7 Unlike Example 1, the dried precursor powder was directly subjected to Joule heating without tableting, and was designated as NFS-J-15.

[0053] Comparative Example 8 Referring to the embodiment of CN119208581A, the sodium ferric sulfate precursor was subjected to rapid heat treatment (approximately 800°C, 1.5s) under a protective atmosphere, but without using Joule heating self-heating and without a tableting densification step, and is designated as NFS-J-16.

[0054] The cathode materials prepared in Examples 1-8 and Comparative Examples 1-8 were characterized, and the results are as follows: Figure 1-6 As shown.

[0055] Figure 1 The X-ray powder diffraction (XRD) patterns of the cathode materials prepared in Examples 1-8 and Comparative Examples 1-2 are shown below. Figure 1 It can be seen that Joule heating, under appropriate impact time and temperature, synthesizes the pure phase in both the Joule heating and tubular furnace heating processes, proving that different calcination methods do not change the structure of sodium ferric sulfate.

[0056] Figure 2 The X-ray powder diffraction (XRD) patterns of the cathode materials prepared in Comparative Examples 3-6 are shown below. Figure 2 As can be seen from the data, during Joule heating, neither excessively long nor short impact time, nor excessively high or low calcination temperature, resulted in the synthesis of a pure phase. This indicates that the 500-700℃ range is an optimized range that has been screened through a large number of experiments and can be easily obtained by non-technical personnel through limited experiments.

[0057] Figure 3 The images show the Fourier Transform Infrared (FTIR) spectra of the cathode materials prepared in Examples 1-3 and Comparative Example 1. Figure 3 As can be seen from this, the Na synthesized by Joule heating and tubular furnace calcination 2.6 Fe 1.7 (SO4)3 cathode material, wavelength at 1056 cm⁻¹ -1 With 992cm -1 SO4 was found nearby 2- The peak of the functional group is at 628cm. -1 and 596cm -1 The presence of Fe-O peaks indicates the successful synthesis of the material.

[0058] Figure 4 and Figure 5 The images show SEM images of the cathode materials prepared in Examples 1-4 and Comparative Examples 1-2, respectively. It can be seen that the particles synthesized by the Joule heating method of this invention are uniform in size and exhibit a uniform blocky structure, which increases the contact area with the electrolyte. Furthermore, the Na... 2.6 Fe 1.7 (SO4)3 exists as particles of hundreds of nanometers in size, while Comparative Examples 1 and 2 were calcined in a tube furnace, resulting in more severe agglomeration between particles in the synthesized products, making it difficult for the electrolyte to fully wet the particles inside the agglomerates.

[0059] Figure 6 The EPR plots for Example 1 and Comparative Example 1 show a peak at g=2.003, which represents oxygen vacancies. The stronger the peak, the higher the oxygen vacancy content. It is clear that the material synthesized by Joule heating has a significantly increased oxygen vacancy content. The formed oxygen vacancies lead to local lattice distortion, widen the ion channels, and the resulting defect energy level can increase the electronic state density near the Fermi level, thereby enabling the material to obtain excellent ionic / electronic conductivity and improve rate and cycle performance.

[0060] Figure 7 The diagram shows the apparatus used during calcination in Examples 1-8 and Comparative Examples 3-6. It can be seen that the tableted samples were placed between two layers of carbon cloth.

[0061] Comparative Example 7, which was not tableted, produced powder splashing during calcination, making it impossible to synthesize the sample. This indicates that tableting facilitates uniform heat conduction and achieves a uniform reaction.

[0062] Performance testing The following steps are taken to prepare coin cells using the cathode materials from the above embodiments and comparative examples: The positive electrode material, Super P, and polyvinylidene fluoride (PVDF) were mixed in a mass ratio of 8:1:1 and thoroughly ground. These three materials were then dispersed in 800 μL of N-methylpyrrolidone and homogenized for 30 min to prepare a conductive slurry. This slurry was then uniformly coated onto an aluminum foil current collector at a height of 150 μm under a dry environment. After vacuum drying at 100℃ for 12 h, the positive electrode sheet was obtained. The solvent was removed, and the sheet was cut into 10 mm diameter discs. The negative electrode used metallic sodium, the separator was a GF / D glass fiber filter paper membrane, and the electrolyte was 1 M sodium perchlorate (NaClO4), dissolved in a 1:1 volume ratio mixture of ethylene carbonate (EC) and propylene carbonate (PC), with 5% fluoroethylene carbonate (FEC) added. Electrochemical performance was tested at room temperature after assembling the coin cell. The test results are as follows: Figure 8-10 As shown in Table 1.

[0063] Figure 8 The graphs show the rate performance and cycling performance at 1C current density of sodium-ion batteries prepared using the cathode materials of Examples 1-4 and Comparative Example 1. It can be seen that the Na synthesized by Joule heating... 2.6 Fe 1.7 (SO4)3 still has a specific capacity of nearly 80 mAh / g at a high rate of 50C, which far exceeds the high rate performance of the tube furnace. After 200 cycles at 1C, the capacity retention rate is about 92%, indicating that the Joule thermal calcination ultrafast synthesis method can effectively improve the high rate performance and capacity of sodium ferric sulfate cathode material.

[0064] Figure 9 The left figure shows the charge-discharge curves of Example 1 and Comparative Example 1 at a current density of 0.5C for the first two cycles. The capacity of Example 1 is close to 100 mAh / g, while that of Comparative Example 1 is 90 mAh / g, indicating that the material synthesized by Joule heating has superior performance. The right figure shows the charge-discharge curves of Examples 1-8 and Comparative Examples 1-2 at a current density of 1C. The lower capacity of Example 8 is due to the use of carbon nanotubes as the carbon material, which was not dispersed. This does not prove any limitation of the synthesis method. The performance of the other materials synthesized by Joule heating is generally higher than that of the comparative examples.

[0065] Figure 10 The GITT curves for sodium-ion batteries prepared with the cathode materials of Example 1 and Comparative Example 1 are shown. The sodium-ion diffusion coefficient of Example 1 is 10. -10 -10 -13 .5 cm 2 ·s -1 The sodium ion diffusion coefficient of Comparative Example 1 is 10. -11 -10 - 15.5 cm 2 ·s -1This further demonstrates that Example 1 exhibits excellent sodium ion migration kinetics.

[0066] Table 1. Electrochemical performance of coin cells prepared from the cathode materials of the examples and comparative examples.

[0067] Based on the data in Table 1, it can be seen that the calcination temperature, carbon material, and Na / Fe ratio all affect the performance of the material in the Joule heating synthesis. However, overall, the sodium ferric sulfate material synthesized by Joule heating has better performance and exhibits better kinetics than the sodium ferric sulfate material synthesized by tube furnace.

[0068] Furthermore, compared with existing technologies that also involve rapid heat treatment (Comparative Example 8), the present invention combines tablet densification with Joule heating self-heating mode, resulting in a significantly higher concentration of oxygen vacancies and superior ultra-high rate performance. Due to process limitations, the high rate performance of Comparative Example 8 could not be effectively tested, resulting in a qualitative difference in technical effects.

[0069] In summary, the sodium ferric sulfate material synthesized by Joule heating exhibits good electrochemical performance, with a significant improvement in high-rate performance. Furthermore, this invention is low-cost, fast, energy-efficient, and highly efficient, making it a worthwhile and effective method to consider in the preparation of sodium-ion battery materials.

[0070] The embodiments described above are merely illustrative of several implementations of the present invention, and while the descriptions are specific and detailed, they should not be construed as limiting the scope of the invention. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of the present invention, and these modifications and improvements all fall within the scope of protection of the present invention. Therefore, the scope of protection of the present invention should be determined by the appended claims.

Claims

1. A method for preparing a high-rate performance sodium ferric sulfate cathode material, characterized in that, Includes the following steps: (1) The iron source, sodium source and carbon source are wet ball milled under an inert atmosphere, and after drying and grinding, a uniformly mixed precursor powder is obtained; (2) The precursor powder is pressed into a compacted precursor sheet. (3) The precursor sheet is subjected to Joule heating under an inert atmosphere, wherein the calcination conditions are: impact heating time 3-7s, calcination temperature 500-700℃, and holding time ≤1s, to obtain sodium iron sulfate cathode material with oxygen vacancies.

2. The preparation method according to claim 1, characterized in that, The Joule heating impact heating time is 5-6 s, the calcination temperature is 550-700℃, and the heating rate is 100-200℃ / s.

3. The preparation method according to claim 1, characterized in that, In step (1), the iron source is at least one of ferrous sulfate heptahydrate, ferrous sulfate monohydrate, or anhydrous ferrous sulfate; the sodium source is at least one of sodium sulfate or anhydrous sodium sulfate; and the carbon source is at least one of carbon nanotubes, acetylene black, or Ketjen black. The Na:Fe molar ratio in the sodium source and iron source is 1.2-1.6:1; The carbon source accounts for 5%-10% of the total mass of the iron source, sodium source, and carbon source.

4. The preparation method according to claim 1, characterized in that, In step (1), the ball milling speed is 300-400 r / min, the ball milling time is 10-14 h, the vacuum drying temperature is 60-80 ℃, and the time is 3-8 h.

5. The preparation method according to claim 1, characterized in that, In step (1), the grinding media used in the ball mill are all zirconia balls, with the ratio of 5mm diameter zirconia balls to 3mm diameter zirconia balls being 1:2, and the ball-to-material ratio being set to 10:1-20:

1.

6. The preparation method according to claim 1, characterized in that, In step (2), the tableting process involves placing the precursor powder in a tablet press and pressing it at a pressure of 5-10 MPa for 3-5 seconds.

7. A sodium ferric sulfate cathode material, characterized in that, The material is prepared by the method according to any one of claims 1-6, and the oxygen vacancy content is characterized by the signal intensity of the electron paramagnetic resonance spectrum at g=2.

003.

8. The sodium ferric sulfate cathode material according to claim 7, characterized in that, The material has a discharge specific capacity of not less than 75 mAh / g at a 50C rate, and the sodium ion diffusion coefficient of the material is 10. -10 -10 -14 cm 2 ·s -1 .

9. An electrode sheet, characterized in that, Includes the cathode material as described in claim 7 or 8.

10. A sodium-ion battery, comprising a positive electrode, a negative electrode, a separator, and an electrolyte, characterized in that, The positive electrode uses the electrode sheet as described in claim 9.

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