Preparation method and application of carbon shell coated sodium iron phosphate positive electrode material with water-based binder

Abstract

The application relates to a preparation method and application of a carbon shell coated sodium iron phosphate sodium pyrophosphate positive electrode material suitable for water-based binders, and relates to the preparation method and application of a sodium ion battery positive electrode material. The technical problem that the existing sodium iron phosphate sodium pyrophosphate will dissolve and cause a side reaction when a water-based binder is used is solved. The method comprises the following steps: firstly, mixing a sodium source, an iron source, a phosphorus source and a carbon source in deionized water; secondly, spray drying to prepare a precursor; and thirdly, high-temperature solid phase treatment to obtain a carbon shell coated sodium iron phosphate sodium pyrophosphate positive electrode material. The carbon shell coated sodium iron phosphate sodium pyrophosphate positive electrode material is used to prepare an electrode sheet together with a water-based binder, the maximum loading of the obtained electrode sheet reaches 10.3 mg / cm 2 , the first circle discharge maximum specific capacity reaches 118.6 mAh / g under the condition of 0.1C discharge, and reaches 101 mAh / g under the condition of 1C discharge, and can be used in the field of sodium ion batteries.

Description

Technical Field

[0001] This invention belongs to the field of battery material preparation technology, specifically relating to a method for preparing sodium iron pyrophosphate cathode material. Background Technology

[0002] Sodium iron pyrophosphate (SOP) is a sodium-ion battery cathode material with advantages such as high theoretical specific capacity, long cycle life, environmental friendliness, and low production cost. For various new energy sources, such as solar, wind, and tidal energy, which cannot be directly fed into the grid for direct utilization, large-scale energy storage devices are needed for energy recovery. However, due to the rising price of lithium-ion batteries in recent years, the cost of such large-scale energy storage devices has become a significant issue. Furthermore, the traditional use of polyvinylidene fluoride (PVDF) as a binder requires N-methylpyrrolidone (NMP) as a solvent; however, NMP is biotoxic and a volatile organic compound. Its production, use, and recycling processes pose health and environmental risks and have high recycling costs. Therefore, an environmentally friendly and inexpensive cathode material and production process are needed that are compatible with such large-scale energy storage power stations.

[0003] Aqueous binders such as sodium carboxymethyl cellulose, polyacrylic acid, and sodium alginate use deionized water as a solvent, which greatly reduces environmental and cost issues. However, when using aqueous binders, the cathode material may dissolve and undergo side reactions, affecting the material's electrochemical performance. Therefore, adapting materials to aqueous binders is a challenge that needs to be overcome. Summary of the Invention

[0004] This invention aims to address the technical problem of dissolution and side reactions that occur when using aqueous binders in existing cathode materials such as sodium iron pyrophosphate. It provides a method for preparing and applying a carbon-coated sodium iron pyrophosphate cathode material adapted to aqueous binders. This method utilizes spray drying combined with high-temperature solid-phase processing to prepare a high-carbon-content carbon-coated sodium iron pyrophosphate material. Liquid-phase reaction is used to disperse the elements at the molecular level, and spray drying achieves carbon coating. This carbon coating prevents dissolution and side reactions caused by aqueous binders and enables high loading capacity.

[0005] The preparation method of the carbon-shell coated sodium iron pyrophosphate cathode material adapted to an aqueous binder of the present invention is carried out according to the following steps:

[0006] 1. Add carbon source and iron source to deionized water and mix evenly to obtain a mixture; then add the mixture, sodium source and phosphorus source to deionized water, heat and stir to mix the materials evenly to obtain a dispersion.

[0007] 2. The dispersion is spray-dried to obtain spherical particles, which is the precursor; or the dispersion is spray-dried to obtain spherical particles, which are then pressed into flakes to obtain the precursor.

[0008] 3. Place the precursor in a tube furnace under a protective atmosphere, heat it to 300-350℃ at a heating rate of 2-5℃ / min, hold it for 4-8 hours, and then cool it naturally to room temperature to obtain a pre-burned body. Place the pre-burned body or the pre-burned body ground into powder in a tube furnace under a protective atmosphere, heat it to 500-550℃ at a heating rate of 2-5℃ / min, hold it for 8-12 hours, and then cool it naturally to room temperature to obtain a carbon-shell coated sodium iron pyrophosphate cathode material suitable for water-based binders.

[0009] Furthermore, the sodium source mentioned in step one is sodium pyrophosphate (Na4P2O7) and / or sodium dihydrogen phosphate.

[0010] Furthermore, the iron source mentioned in step one is one or more of ferric chloride, ferric nitrate nonahydrate, and ferric oxide.

[0011] Furthermore, the phosphorus source mentioned in step one is one or more of ammonium dihydrogen phosphate (NH4H2PO4), sodium dihydrogen phosphate, sodium pyrophosphate, and diammonium hydrogen phosphate.

[0012] Furthermore, the carbon source mentioned in step one is anhydrous citric acid.

[0013] Furthermore, the molar ratio of sodium source, iron source and phosphorus source mentioned in step one is (1~2):3:(1~2). If the molar ratio of sodium source to iron source is too high or too low, impurity phases are easily generated, affecting the performance of the product. Under certain conditions, the product obtained by limiting the molar ratio of sodium source to iron source is purer.

[0014] Furthermore, the mass of the carbon source mentioned in step one is 30% to 50% of the total mass of the sodium source, iron source, phosphorus source, and carbon source. A high carbon content can improve the conductivity of the material and reduce direct contact between the material and the solvent water.

[0015] Furthermore, the mixture described in step one is a sol-gel, solution, or dispersion. Sol-gel treatment of the iron source and carbon source can make the materials mix more uniformly and improve the granulation effect; simply mixing the iron source and carbon source directly yields a solution or dispersion.

[0016] Furthermore, the heating and stirring described in step one involves heating to 60-90°C and maintaining it for 30-60 minutes under magnetic stirring at a speed of 400-600 r / min.

[0017] Furthermore, the spray drying described in step two is carried out under conditions where the inlet temperature is 200~240℃ and the outlet temperature is 120~125℃.

[0018] Furthermore, the pressing described in step two is carried out under a pressure of 20~25MPa, with the aim of making the materials more closely contacted and improving the crystallinity of the sintering process.

[0019] Furthermore, the protective atmosphere described in step three is an argon atmosphere or a hydrogen-argon mixture atmosphere.

[0020] Furthermore, the grinding process described in step three takes 20 to 30 minutes.

[0021] The application of the carbon-shell coated sodium iron pyrophosphate cathode material adapted to aqueous binders prepared by the above method is to use this material to prepare the cathode of sodium-ion batteries using aqueous binders.

[0022] The method for preparing electrode sheets using the carbon shell-coated sodium iron pyrophosphate cathode material with the above-mentioned water-compatible binder is as follows:

[0023] 1. Accurately weigh the carbon shell coated sodium iron pyrophosphate cathode material, conductive agent, aqueous binder, and deionized water in a mass ratio of 7:2:1:1; wherein the aqueous binder is CMC, PAA, or / and SA.

[0024] 2. First, dissolve the aqueous binder in deionized water and stir continuously for 30 minutes. Then, add the conductive agent and stir continuously for 15 minutes. Finally, add the carbon shell-coated sodium iron pyrophosphate cathode material that is compatible with the aqueous binder and stir for 10 minutes to obtain the slurry.

[0025] 3. Use a scraper to coat the slurry onto the aluminum foil to a thickness of 150μm; then place it in a vacuum oven and vacuum dry it for 12h at a temperature of 65℃ and a pressure of -0.1Pa to obtain the electrode sheet.

[0026] Furthermore, the conductive agent mentioned in step one is conductive carbon black (Super P) or hydroxylated multi-walled carbon nanotubes.

[0027] Furthermore, in step three, the loading of the carbon shell-coated sodium iron pyrophosphate cathode material adapted to the aqueous binder on the electrode sheet is 1~12 mg / cm³. 2 .

[0028] The advantages of this invention over the prior art are as follows:

[0029] (1) Carbon coating is carried out on the cathode material with high carbon content without affecting the material structure and performance to improve the waterproof performance of the material, reduce the direct contact between the material and the solvent water, thereby reducing the dissolution of the cathode material and the side reactions caused by the dissolution phenomenon, and increasing the conductivity of the material.

[0030] (2) A multi-step dissolution method is adopted to enhance the degree of dispersion and remove heteroatoms to the greatest extent possible, turning the original emulsion dispersion into a clear and transparent solution, achieving molecular-level dispersion, thereby further improving the purity and crystallinity of the final cathode material and improving the material performance.

[0031] (3) By pressing before solid-state sintering, the contact between particles is increased and the carbon source is more tightly dispersed around the material. Annealing and grinding remove the water of crystallization in the material and increase the degree of carbonization and dispersion, reducing the occurrence of agglomeration, thereby further enhancing the performance.

[0032] (4) Increase the load capacity by increasing the electrode thickness. Due to the superior conductivity of water-based binders compared to PVDF, the surface energy density of the material can be increased without affecting the specific capacity. Attached Figure Description

[0033] Figure 1 The first charge-discharge curve of the sodium iron pyrophosphate cathode material prepared in Example 1 at a rate of 0.1 C is shown.

[0034] Figure 2 The XRD pattern of the sodium iron pyrophosphate cathode material prepared in Example 1 is shown below.

[0035] Figure 3 The first charge-discharge curves of the sodium iron pyrophosphate cathode materials prepared in Example 2 and Comparative Example 1 at a rate of 0.1 C are shown.

[0036] Figure 4 The first charge-discharge curve of the sodium iron pyrophosphate cathode prepared in Comparative Example 2 at a rate of 0.1 C is shown.

[0037] Figure 5 The first charge-discharge curve of the sodium iron pyrophosphate cathode material prepared in Comparative Example 3 at a rate of 0.1 C is shown.

[0038] Figure 6 The first charge-discharge curve of the sodium iron pyrophosphate cathode material prepared in Comparative Example 4 at a rate of 0.1 C is shown.

[0039] Figure 7 The first charge-discharge curve of sodium iron pyrophosphate cathode material of Comparative Example 5 at a rate of 0.1 C is shown. Detailed Implementation

[0040] The beneficial effects of the present invention will be verified using the following examples.

[0041] Example 1: The preparation method of the carbon shell coated sodium iron pyrophosphate cathode material with water-based binder in this example is carried out according to the following steps:

[0042] 1. Add 3.8424g of anhydrous citric acid and 2.433g of ferric chloride to 30ml of deionized water and stir at 500rpm for 4h at 80℃ to obtain a gel. Then place the gel in a forced-air oven and dry at 80℃ for 12h to obtain a mixture. Add the mixture to 150ml of deionized water and stir until completely dissolved. Then add 1.1503g of ammonium dihydrogen phosphate and 1.3295g of sodium pyrophosphate. Heat to 80℃ and maintain for 60min under magnetic stirring at 500r / min to mix the materials evenly to obtain a dispersion.

[0043] 2. The dispersion is added to the spray drying equipment and spray dried under the conditions of inlet temperature of 200℃ and outlet temperature of 120℃ to obtain spherical particles. The spherical particles are then pressed into flakes under a pressure of 20MPa to obtain the precursor.

[0044] 3. The precursor is placed in a tube furnace under an argon atmosphere and heated to 300℃ at a heating rate of 2℃ / min. After holding at this temperature for 4 hours, it is naturally cooled to room temperature to obtain a pre-burned body. The pre-burned body is ground for 20 minutes, and the resulting powder is then placed in a tube furnace under an argon atmosphere and heated to 500℃ at a heating rate of 2℃ / min. After holding at this temperature for 8 hours, it is naturally cooled to room temperature to obtain a carbon-shell coated sodium iron pyrophosphate cathode material suitable for aqueous binders.

[0045] The electrode sheet was prepared using the carbon-shell coated sodium iron pyrophosphate cathode material with an adaptable aqueous binder prepared in Example 1 as the active material. The specific steps are as follows:

[0046] 1. Accurately weigh the active material, conductive agent SuperP, binder sodium carboxymethyl cellulose, and deionized water according to a mass ratio of 7:2:1:1;

[0047] 2. First, dissolve sodium carboxymethyl cellulose in deionized water and stir continuously for 30 minutes. Then, add the conductive agent SuperP and stir continuously for 15 minutes. Finally, add the active material and stir for 10 minutes to obtain the slurry.

[0048] 3. The slurry is coated onto aluminum foil using a scraper to a thickness of 150 μm; then it is placed in a vacuum oven and vacuum dried for 12 h at a temperature of 65℃ and a pressure of -0.1 Pa to obtain a sodium iron pyrophosphate cathode sheet. The loading of the sodium iron pyrophosphate cathode material coated with a water-based binder on the electrode sheet is 2 mg / cm³. 2 .

[0049] After accurately weighing the electrode sheets, the positive electrode shell, sodium iron pyrophosphate positive electrode sheet, glass fiber separator, sodium metal negative electrode sheet, and negative electrode shell were assembled in sequence in a glove box, and electrolyte was added before pressing to form a coin cell. The electrolyte consisted of sodium perchlorate as the electrolyte, ethylene carbonate and propylene carbonate in a 1:1 volume ratio as the solvent, and fluoroethylene carbonate at a volume fraction of 5% as an additive.

[0050] Electrochemical performance tests were conducted on the Xinwei all-in-one machine, with a test voltage range of 1.5~4.0V. Figure 1 The image shows the initial charge-discharge curves of the sodium-ion battery cathode material (sodium iron pyrophosphate) prepared in Example 1 at a rate of 0.1 C. From... Figure 1 It can be seen that at 25 ℃, the initial discharge specific capacity at a 0.1 C rate is 112.53 mAh / g, and the median discharge voltage is 2.9V.

[0051] The carbon content of the carbon shell-coated sodium iron pyrophosphate cathode material prepared in Example 1 with an aqueous binder was tested using a Wuxi high-speed HIR944 carbon-sulfur analyzer. The results showed that the mass percentage of carbon in the cathode material was as high as 13.01%, indicating that the high carbon content can enable sodium iron pyrophosphate to exhibit excellent performance without affecting the normal crystallization of the material.

[0052] Figure 2 The image shows the refined XRD pattern of the sodium iron pyrophosphate cathode material for sodium-ion batteries prepared in Example 1. The pattern confirms the successful synthesis of sodium iron pyrophosphate. It further demonstrates that the superior performance of the high-carbon-content coated sodium iron pyrophosphate is due to the coating layer's ability to isolate the solvent water.

[0053] Example 2: This example uses ferric nitrate nonahydrate as the iron source. The preparation method of the carbon shell-coated sodium iron pyrophosphate cathode material with a suitable aqueous binder in this example is carried out according to the following steps:

[0054] 1. Add 3.8424g of anhydrous citric acid and 6.06g of ferric nitrate nonahydrate to 30ml of deionized water and stir at 500rpm for 4h at 80℃ to obtain a gel. Then place the gel in a forced-air oven and dry at 80℃ for 12h to obtain a mixture. Add the mixture to 150ml of deionized water and stir until completely dissolved. Then add 1.1503g of ammonium dihydrogen phosphate and 1.3295g of sodium pyrophosphate. Heat to 80℃ and maintain for 60min under magnetic stirring at 500r / min to mix the materials evenly to obtain a dispersion.

[0055] 2. The dispersion is added to the spray drying equipment and spray dried under the conditions of inlet temperature of 200℃ and outlet temperature of 120℃ to obtain spherical particles. The spherical particles are then pressed into flakes under a pressure of 20MPa to obtain the precursor.

[0056] 3. The precursor is placed in a tube furnace under an argon atmosphere and heated to 300℃ at a heating rate of 2℃ / min. After holding at this temperature for 4 hours, it is naturally cooled to room temperature to obtain a pre-burned body. The pre-burned body is ground for 20 minutes, and the resulting powder is then placed in a tube furnace under an argon atmosphere and heated to 500℃ at a heating rate of 2℃ / min. After holding at this temperature for 8 hours, it is naturally cooled to room temperature to obtain a carbon-shell coated sodium iron pyrophosphate cathode material suitable for aqueous binders.

[0057] The electrode sheet was prepared using the carbon-shell coated sodium iron pyrophosphate cathode material with an adaptable aqueous binder prepared in Example 2 as the active material. The specific steps are as follows:

[0058] 1. Accurately weigh the active material, conductive agent SuperP, water-based binder sodium carboxymethyl cellulose, and deionized water according to a mass ratio of 7:2:1:1;

[0059] 2. First, dissolve the water-based binder sodium carboxymethyl cellulose in deionized water and stir continuously for 30 minutes. Then, add the conductive agent SuperP and stir continuously for 15 minutes. Finally, add the active material and stir for 10 minutes to obtain the slurry.

[0060] 3. The slurry is coated onto aluminum foil using a scraper to a thickness of 150 μm; then it is placed in a vacuum oven and vacuum dried for 12 h at a temperature of 65℃ and a pressure of -0.1 Pa to obtain a sodium iron pyrophosphate cathode sheet. The loading of the sodium iron pyrophosphate cathode material coated with a water-based binder on the electrode sheet is 2 mg / cm³. 2 .

[0061] After accurately weighing the electrode sheets, the positive electrode shell, sodium iron pyrophosphate positive electrode sheet, glass fiber separator, sodium metal negative electrode sheet, and negative electrode shell were assembled in sequence in a glove box, and electrolyte was added before pressing to form a coin cell. The electrolyte consisted of sodium perchlorate as the electrolyte, ethylene carbonate and propylene carbonate in a 1:1 volume ratio as the solvent, and fluoroethylene carbonate at a volume fraction of 5% as an additive.

[0062] Electrochemical performance tests were conducted on the Xinwei all-in-one machine, with a test voltage range of 1.5~4.0V. Figure 3 The image shows the initial charge-discharge curve of the sodium-ion battery cathode material (sodium iron pyrophosphate) prepared in Example 2 at a rate of 0.1 C. Figure 3As shown, the initial discharge specific capacity at 0.1 C rate was 110.51 mAh / g, and the median discharge voltage was 2.9V. The initial discharge specific capacity of Example 2 was slightly worse than that of Example 1. This is because ferric nitrate nonahydrate was used as the iron source, and the residual nitrate ions would cause oxidation of the sintered material, resulting in a slight decrease in performance. In contrast, Example 1 used ferric chloride as the iron source, which did not cause oxidation by nitrate ions, thus improving performance.

[0063] Comparative Example 1: This comparative example does not include presol-gel treatment before spray drying. The specific preparation method of the phosphoric acid pyrophosphate cathode material is as follows:

[0064] 1. Add 3.8424g of anhydrous citric acid, 6.06g of ferric nitrate nonahydrate, 1,1503g of ammonium dihydrogen phosphate and 1,3295g of sodium pyrophosphate to 150ml of deionized water. Heat to 80℃ and maintain for 60 min under magnetic stirring at 500 r / min to mix the materials evenly and obtain a dispersion.

[0065] 2. The dispersion is added to the spray drying equipment and spray dried under the conditions of inlet temperature of 200℃ and outlet temperature of 120℃ to obtain spherical particles. The spherical particles are then pressed into flakes under a pressure of 20MPa to obtain the precursor.

[0066] 3. The precursor is placed in a tube furnace under an argon atmosphere and heated to 300℃ at a heating rate of 2℃ / min. After holding at this temperature for 4 hours, it is naturally cooled to room temperature to obtain a pre-burned body. The pre-burned body is ground for 20 minutes, and the resulting powder is then placed in a tube furnace under an argon atmosphere and heated to 500℃ at a heating rate of 2℃ / min. After holding at this temperature for 8 hours, it is naturally cooled to room temperature to obtain sodium iron pyrophosphate cathode material.

[0067] Using the sodium iron pyrophosphate cathode material with a water-based binder prepared in Comparative Example 1 as the active material, electrode sheets and coin cells were prepared using the same method as in Example 1. Electrochemical performance was tested on a Newway integrated instrument, with a test voltage range of 1.5~4.0V. The initial charge-discharge curve of the sodium ion sodium iron pyrophosphate cathode material prepared in Comparative Example 1 at a 0.1C rate was also plotted. Figure 3 From Figure 3 As can be seen, since Comparative Example 1 omitted the special sol-gel treatment before spray drying compared to Example 2, the dispersion that was originally a clear and transparent solution after treatment was transformed into an emulsion, which is not conducive to the dispersion of various raw materials and also affects the granulation effect of the subsequent spray drying method. Therefore, the discharge specific capacity is lower than that of Example 2, with an initial discharge specific capacity of 107.28 mAh / g at a rate of 0.1 C and a median discharge voltage of 2.9 V.

[0068] Comparative Example 2: This comparative example prepared a high-load, high-performance cathode sheet. The carbon-shell coated sodium iron pyrophosphate cathode material with an aqueous binder prepared in Example 1 was used as the active material to prepare the electrode sheet. The specific steps are as follows:

[0069] 1. Accurately weigh the active material, conductive agent SuperP, binder sodium carboxymethyl cellulose, and deionized water according to a mass ratio of 7:2:1:1;

[0070] 2. First, dissolve sodium carboxymethyl cellulose in deionized water and stir continuously for 30 minutes. Then, add the conductive agent SuperP and stir continuously for 15 minutes. Finally, add the active material and stir for 10 minutes to obtain the slurry.

[0071] 3. The slurry was coated onto aluminum foil using a scraper to a thickness of 300 μm; then placed in a vacuum oven and vacuum dried for 12 h at 65℃ and -0.1 Pa to obtain a sodium iron pyrophosphate cathode sheet. The loading of the sodium iron pyrophosphate cathode material coated with a water-based binder on the electrode sheet was 10.3 mg / cm³. 2 .

[0072] The electrode sheet prepared in Comparative Example 2 was used to prepare coin cells using the same method as in Example 1. Electrochemical performance was tested on a Newway integrated analyzer, with a test voltage range of 1.5–4.0 V. Figure 4 The first charge-discharge curve of the sodium-ion battery positive electrode sheet prepared in Comparative Example 2 at a rate of 0.1 C is shown. The active material loading of the electrode sheet prepared in Comparative Example 2 is 10.3 mg / cm³. 2 Because the aqueous binder has better conductivity than traditional PVDF, the high-capacity electrode has little impact on the material's specific capacity. The initial discharge specific capacity is 104.9 mAh / g, compared to 112.53 mAh / g in Example 1, a decrease of only 6.7%. The median discharge voltage is 2.9 V. The high-performance, high-capacity electrode demonstrates the excellent performance of the high-carbon-content coated sodium iron pyrophosphate cathode material combined with the aqueous binder.

[0073] Comparative Example 3: In this comparative example, the phosphorus source ammonium dihydrogen phosphate and the sodium source sodium pyrophosphate were replaced with sodium dihydrogen phosphate. The specific preparation method of the phosphoric acid pyrophosphate cathode material is as follows:

[0074] 1. Add 3.8424g of anhydrous citric acid and 2.433g of ferric chloride to 30ml of deionized water and stir at 500rpm for 4h at 80℃ to obtain a gel. Then place the gel in a forced-air oven and dry at 80℃ for 12h to obtain a mixture. Add the mixture to 150ml of deionized water and stir until completely dissolved. Then add 2.392g of sodium dihydrogen phosphate as a sodium source and phosphorus source. Pyrophosphate is generated through the dehydration condensation reaction of phosphate. Heat to 80℃ under magnetic stirring at 500 r / min and maintain for 60 min to mix the materials evenly to obtain a dispersion.

[0075] 2. The dispersion is added to the spray drying equipment and spray dried under the conditions of inlet temperature of 200℃ and outlet temperature of 120℃ to obtain spherical particles. The spherical particles are then pressed into flakes under a pressure of 20MPa to obtain the precursor.

[0076] 3. The precursor is placed in a tube furnace under an argon atmosphere and heated to 300℃ at a heating rate of 2℃ / min. After holding at this temperature for 4 hours, it is naturally cooled to room temperature to obtain a pre-burned body. The pre-burned body is ground for 20 minutes, and the resulting powder is then placed in a tube furnace under an argon atmosphere and heated to 500℃ at a heating rate of 2℃ / min. After holding at this temperature for 8 hours, it is naturally cooled to room temperature to obtain sodium iron pyrophosphate cathode material.

[0077] In this comparative example, the phosphorus source ammonium dihydrogen phosphate and the sodium source sodium pyrophosphate were replaced with sodium dihydrogen phosphate. The active material was sodium iron pyrophosphate prepared in Comparative Example 3. Electrode sheets and button half-cells were prepared using the same method as in Example 1. Electrochemical performance was tested on a Newway integrated machine with a test voltage range of 1.5~4.0V. Figure 5 The first charge-discharge curves of the sodium iron pyrophosphate cathode material prepared in Comparative Example 3 at a rate of 0.1 C are shown. Figure 6 It can be seen that the initial discharge specific capacity at 0.1C rate is 78.22 mAh / g, and the median discharge voltage is 2.9 V. Since Comparative Example 3 only uses sodium dihydrogen phosphate as both the iron and sodium source, pyrophosphate ions need to be obtained through phosphate condensation. This makes it difficult to precisely control the ratio, thus affecting the purity of the final material. This may result in the formation of electrochemically inactive sodium iron phosphate cathode material or sodium iron pyrophosphate cathode material with a small theoretical capacity, thereby affecting the performance of the sodium iron pyrophosphate material. Therefore, the initial charge-discharge capacity of Comparative Example 3 is lower than that of Example 1.

[0078] Comparative Example 4: This comparative example omits the annealing and grinding process after holding at 300℃. The pre-sintered body is placed in a tube furnace and directly heated for the next sintering step. The specific preparation method of the phosphoric acid pyrophosphate cathode material is carried out according to the following steps:

[0079] 1. Add 3.8424g of anhydrous citric acid and 2.433g of ferric chloride to 30ml of deionized water and stir at 500rpm for 4h at 80℃ to obtain a gel. Then place the gel in a forced-air oven and dry at 80℃ for 12h to obtain a mixture. Add the mixture to 150ml of deionized water and stir until completely dissolved. Then add 1.1503g of ammonium dihydrogen phosphate and 1.3295g of sodium pyrophosphate. Heat to 80℃ and maintain for 60min under magnetic stirring at 500r / min to mix the materials evenly to obtain a dispersion.

[0080] 2. The dispersion is added to the spray drying equipment and spray dried under the conditions of inlet temperature of 200℃ and outlet temperature of 120℃ to obtain spherical particles. The spherical particles are then pressed into flakes under a pressure of 20MPa to obtain the precursor.

[0081] 3. The precursor is placed in a tube furnace under an argon atmosphere and heated to 300℃ at a heating rate of 2℃ / min and held for 4 hours. Then, it is heated to 500℃ at a heating rate of 2℃ / min and held for 8 hours. After natural cooling to room temperature, sodium iron pyrophosphate cathode material is obtained.

[0082] Using the sodium iron pyrophosphate cathode material prepared in Comparative Example 4 as the active material, electrode sheets and coin cells were prepared using the same method as in Example 1. Electrochemical performance was tested on a Newway integrated testing machine, with a test voltage range of 1.5~4.0V. Figure 6 This is a graph showing the initial charge-discharge curves of the sodium iron pyrophosphate cathode material prepared in Comparative Example 4 at a rate of 0.1 C. From... Figure 6 It can be seen that the initial discharge specific capacity at 0.1 C rate is 93.69 mAh / g, and the median discharge voltage is 2.9 V. Because the grinding process after high-temperature sintering at 300℃ was omitted in Comparative Example 4, the water of crystallization was not completely removed, and agglomeration was prone to occur, resulting in the performance not being fully realized, and the initial charge-discharge specific capacity was slightly lower than that of Example 1.

[0083] Comparative Example 5: This comparative example uses commercially available sodium iron pyrophosphate cathode material as the active material. This material is not carbon-coated. Electrode sheets and coin cells were prepared using the same method as in Example 1. Electrochemical performance was tested on a Newway integrated tester, with a test voltage range of 1.5~4.0V. Figure 7 This is the initial charge-discharge curve of the sodium iron pyrophosphate cathode material of Comparative Example 5 at a rate of 0.1C. From... Figure 7It can be seen that the purchased sodium iron pyrophosphate cathode material has an initial discharge specific capacity of 101.83 mAh / g and a median discharge voltage of 2.9 V. Because this commercial sodium iron pyrophosphate cathode material was not carbon-coated, significant dissolution occurred during cathode slurry preparation, leading to structural damage and affecting electrochemical performance.

[0084] This invention introduces a carbon coating layer with a mass percentage of 10% to 20% as a physical barrier to reduce direct contact between the solvent water and the cathode material. Furthermore, the aqueous binder is rich in carboxyl groups, which can enhance the conductivity of the sodium-ion battery cathode material. Increasing the carbon content in the material can minimize the dissolution and side reactions of the material, thereby improving the electrochemical performance of the material.

Claims

1. The application of a carbon-shell coated sodium iron pyrophosphate cathode material adapted to aqueous binders, characterized in that, This application involves using the carbon-shell-coated sodium iron pyrophosphate cathode material to prepare the cathode of a sodium-ion battery with an aqueous binder. The method for preparing the sodium-ion battery cathode sheet with an aqueous binder using the carbon-shell-coated sodium iron pyrophosphate cathode material is as follows:

1. Accurately weigh the carbon shell coated sodium iron pyrophosphate cathode material, conductive agent, aqueous binder, and deionized water in a mass ratio of 7:2:1:1; wherein the aqueous binder is CMC, PAA, or / and SA.

2. First, dissolve the aqueous binder in deionized water and stir continuously for 30 minutes. Then, add the conductive agent and stir continuously for 15 minutes. Finally, add the carbon shell-coated sodium iron pyrophosphate cathode material that is compatible with the aqueous binder and stir for 10 minutes to obtain the slurry.

3. The slurry is coated onto aluminum foil using a scraper to a thickness of 150 μm; then placed in a vacuum oven and vacuum-dried for 12 hours at 65°C and a vacuum gauge pressure of -0.1 MPa to obtain the electrode sheet; the loading of the carbon shell-coated sodium iron pyrophosphate cathode material with an aqueous binder on the electrode sheet is 1~12 mg / cm³. 2 ; The preparation method of the carbon shell-coated sodium iron pyrophosphate cathode material adapted to aqueous binders is carried out according to the following steps: (i) Add carbon source and iron source to deionized water and mix evenly to obtain a mixture; the mixture is a sol-gel; then add the mixture, sodium source and phosphorus source to deionized water, heat and stir to mix the materials evenly to obtain a dispersion; wherein the molar ratio of sodium source, iron source and phosphorus source is (1~2):3:(1~2); the mass of carbon source is 30%~50% of the total mass of sodium source, iron source, phosphorus source and carbon source; the sodium source is sodium pyrophosphate, and the phosphorus source is one or more of ammonium dihydrogen phosphate, sodium dihydrogen phosphate and diammonium hydrogen phosphate; or the sodium source is sodium dihydrogen phosphate and the phosphorus source is sodium pyrophosphate; (ii) After spray drying the dispersion to obtain spherical particles, the spherical particles are then pressed into flakes to obtain the precursor; (III) The precursor is placed in a tube furnace under a protective atmosphere and heated to 300-350℃ at a heating rate of 2-5℃ / min. After holding at this temperature for 4-8 hours, it is naturally cooled to room temperature to obtain a pre-burned body. The pre-burned body is ground into powder and placed in a tube furnace under a protective atmosphere and heated to 500-550℃ at a heating rate of 2-5℃ / min. After holding at this temperature for 8-12 hours, it is naturally cooled to room temperature to obtain a carbon shell coated sodium iron pyrophosphate cathode material suitable for water-based binders.

2. The application of the carbon-shell coated sodium iron pyrophosphate cathode material adapted to an aqueous binder according to claim 1, characterized in that, The iron source mentioned in step one is one or more of ferric chloride, ferric nitrate nonahydrate, and ferric oxide.

3. The application of the carbon-shell coated sodium iron pyrophosphate cathode material adapted to an aqueous binder according to claim 1, characterized in that, The carbon source mentioned in step one is anhydrous citric acid.

4. The application of the carbon-shell coated sodium iron pyrophosphate cathode material adapted to an aqueous binder according to claim 1, characterized in that, The heating and stirring described in step one involves heating to 60-90°C and maintaining it for 30-60 minutes under magnetic stirring at a speed of 400-600 r / min.

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