A method for preparing carbon-coated sodium iron phosphate by using iron phosphate slag

Abstract

The application belongs to the technical field of positive electrode materials for sodium ion batteries, and discloses a method for preparing carbon-coated sodium iron pyrophosphate phosphate by using iron phosphorus slag, which comprises the following steps: adding nitric acid to the iron phosphorus slag to dissolve and obtain an acidolysis solution; adding ammonia water to adjust the pH of the acidolysis solution, then adding ammonium oxalate to heat and stir to react, and obtaining reaction liquid A; adding a phosphorus source and a sodium source to the reaction liquid A, then adding a mixed carbon source, stirring and dissolving, and then grinding into a uniform slurry; uniformly spraying and drying the slurry to obtain a precursor; and performing high-temperature sintering on the precursor in a nitrogen atmosphere to obtain carbon-coated sodium iron pyrophosphate phosphate. The iron phosphorus slag after lithium extraction from waste lithium iron phosphate positive electrode material is changed from waste to benefit, iron and phosphorus in the iron phosphorus slag are used to prepare sodium iron pyrophosphate phosphate, and the synergistic effect of the mixed carbon source is used to improve the conductivity and electrochemical performance of the material.

Description

Technical Field

[0001] This invention belongs to the technical field of cathode materials for sodium-ion batteries, specifically relating to a method for preparing carbon-coated sodium iron phosphate pyrophosphate using iron-phosphorus slag. Background Technology

[0002] The booming development of new energy vehicles has led to a surge in power battery sales, resulting in an impending "retirement wave" of used power batteries. The "2022 China Lithium Battery Industry Development Index" shows that the total retired power batteries in 2022 amounted to 34.5 GWh (277,000 tons), and the amount of retired lithium iron phosphate (LFP) power batteries is projected to reach 76.7 GWh (511,000 tons) by 2026. Currently, the recycling of used LFP batteries mainly focuses on the precious lithium metal in the LFP cathode material. The iron-phosphate slag after lithium extraction is treated as solid waste due to its low recycling value, resulting in resource waste.

[0003] Sodium-ion batteries not only have the advantages of abundant and widely distributed sodium resources, low cost, and environmental friendliness, but also have advantages such as good power characteristics, wide temperature range adaptability, and safety performance. Sodium-ion batteries will become a useful supplement to lithium-ion batteries.

[0004] The cathode material accounts for approximately 40% of the total battery cost and plays a crucial role in battery performance. Currently, lithium iron phosphate (LiFePO4) batteries have a high market share. After the batteries are scrapped, the precious metal lithium is mainly recycled, while the main component, iron phosphate, is discarded due to its low recycling value, resulting in resource waste. Polyanionic sodium iron phosphate pyrophosphate cathode material (Na4Fe3(PO4)2(P2O7)) has characteristics such as high theoretical specific capacity, good cycle stability, high thermal stability, adjustable voltage, and adjustable chemical composition, making it one of the most widely studied cathode materials for sodium-ion batteries. Phosphorus and iron are the main elements in the composition of sodium iron phosphate pyrophosphate (Na4Fe3(PO4)2(P2O7)). Using the iron-phosphorus slag after lithium extraction to synthesize sodium iron phosphate pyrophosphate can achieve the recycling of iron-phosphorus slag and reduce material costs, resulting in significant economic and social benefits. Currently, there is no research, domestic or international, on the synthesis and application of lithium-extracted iron-phosphorus slag in the synthesis of polyanionic cathode materials for sodium-ion batteries. Therefore, developing a method for preparing sodium iron phosphate pyrophosphate cathode material from lithium-extracted iron-phosphate slag is of great research significance and application value. Summary of the Invention

[0005] The technical problem to be solved by the present invention is to address the shortcomings of the existing technology by providing a method for preparing carbon-coated sodium iron phosphate pyrophosphate using iron-phosphorus slag. This method turns the iron-phosphorus slag after lithium extraction from waste lithium iron phosphate cathode materials into a useful resource by using the iron and phosphorus in it to prepare sodium iron phosphate pyrophosphate. Then, through the synergistic effect of mixed carbon sources, the conductivity and electrochemical performance of the material are improved.

[0006] To address the technical problem proposed in this invention, this invention provides a method for preparing carbon-coated sodium iron phosphate pyrophosphate using iron-phosphate slag, comprising the following steps:

[0007] 1) Add nitric acid to the iron-phosphorus slag to dissolve it, filter it, and obtain an acid hydrolysate containing iron and phosphorus;

[0008] 2) Add ammonia to the acid hydrolysate to adjust the pH, then add ammonium oxalate, heat and stir to react, and obtain reaction solution A;

[0009] 3) Add phosphorus source and sodium source to reaction solution A, then add carbon source, stir to dissolve, and grind into a uniform slurry;

[0010] 4) The uniform slurry obtained in step 3) is spray-dried to obtain the precursor;

[0011] 5) The precursor was sintered at high temperature in a nitrogen atmosphere to obtain carbon-coated sodium iron pyrophosphate.

[0012] In the above scheme, the iron-phosphorus slag is the iron-phosphorus slag after lithium extraction from waste lithium iron phosphate cathode material, with a phosphorus content of not less than 20%, an iron content of not less than 30%, and a carbon content of not more than 0.01%.

[0013] In the above scheme, the mass concentration of nitric acid is 30-40%.

[0014] In the above scheme, the mass ratio of the iron-phosphorus slag to nitric acid is (2-3):(7-8).

[0015] In the above scheme, the dissolution time in step 1) is 20 to 40 minutes.

[0016] In the above scheme, the mass concentration of the ammonia water is 25-30%.

[0017] In the above scheme, the ammonia water is used to adjust the pH of the acid hydrolysate to 5-6.

[0018] In the above scheme, the molar ratio of ammonium oxalate to iron ions in the acid hydrolysate is (1.5~2):1.

[0019] In the above scheme, the heating temperature in step 2) is 30-40℃ and the stirring time is 30-50min.

[0020] In the above scheme, the phosphorus source is ammonium dihydrogen phosphate.

[0021] In the above scheme, the molar ratio of iron to phosphorus in the phosphorus source adjustment reaction solution A is 1:(1.4~1.5).

[0022] In the above scheme, the sodium source is at least one of sodium acetate, sodium carbonate, and sodium citrate.

[0023] In the above scheme, the sodium source adjusts the molar ratio of iron to sodium in reaction solution A to 1:(1.35~1.4).

[0024] In the above scheme, the carbon source is a mixed carbon source of polydopamine and glucose.

[0025] Furthermore, the mass ratio of polydopamine to glucose is 1:(1-2).

[0026] In the above scheme, the amount of carbon source added is 30-40% of the total mass of phosphorus source and sodium source.

[0027] In the above scheme, the inlet air temperature of the spray dryer is 240-260℃, and the outlet air temperature is 90-100℃.

[0028] In the above scheme, the sintering temperature of the high-temperature sintering is 600-800℃, and the sintering time is 10-12h.

[0029] In the above scheme, the particle size of the carbon-coated sodium iron pyrophosphate is 0.5-20 μm.

[0030] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0031] 1) This invention transforms iron-phosphorus slag solid waste into a valuable resource, efficiently utilizing the iron and phosphorus elements in the slag and greatly increasing its added value. Using iron-phosphorus slag as a raw material, this invention serves as a partial source of iron and phosphorus, directly producing sodium iron phosphate pyrophosphate in one step. The entire process is carried out in a solution environment, with raw materials added according to the stoichiometric ratio of the target substances, and then directly generated. The product can be directly used as a positive electrode material. Compared with other existing technologies that utilize iron-phosphorus slag to produce intermediate products or precursors, this invention eliminates intermediate washing and drying processes, making it not only economical and environmentally friendly, but also providing products with greater application value and prospects.

[0032] 2) This invention uses a mixed carbon source of polydopamine and glucose for coating. The mixture is spray-dried and uniformly coated on the surface of the material. The mixed carbon source is sintered in an inert atmosphere, and the carbon source is decomposed into a conductive carbon layer, which is uniformly coated on the surface of the material. This improves the electrical conductivity and electrochemical performance of the material, while providing an effective buffer for volume changes during the charging and discharging process. The prepared material has excellent rate performance and cycle performance.

[0033] 3) This invention utilizes a mixed carbon source, which represents a significant improvement over a single carbon source. Firstly, because small particles agglomerate during sintering, they require crushing after sintering before use as cathode materials. When glucose is used as a single carbon source, this crushing process easily causes the carbon coating to detach or be damaged, resulting in incomplete carbon coating on the cathode material surface. This, in turn, affects the material's conductivity and electrochemical performance. However, when using a mixed carbon source of polydopamine and glucose, polydopamine, with its excellent adhesion and self-polymerization properties, can increase the adhesion and bonding of the carbon coating formed on the cathode material surface. The glucose and polydopamine composite carbon source is more densely packed. First, the dense coating on the surface of the cathode material makes it less prone to falling off during breakage and packaging transportation, which is beneficial to improving the conductivity and electrochemical performance of the cathode material. Second, the use of polydopamine can further enhance the reducibility of the mixed carbon source during high-temperature carbonization, which is beneficial to the reduction of trivalent iron to divalent sodium iron pyrophosphate cathode material, thus facilitating the formation of the target product. Third, the carbon layer converted from polydopamine is nitrogen-doped carbon. The high-temperature sintering process of polydopamine forms a nitrogen-doped carbon layer. The defect structure introduced by the nitrogen-doped carbon layer accelerates electron conduction, enhances the electronic conductivity and sodium ion diffusion performance of the carbon layer, and improves the rate performance of the material. Attached Figure Description

[0034] Figure 1 The rate performance diagram shows the sodium-ion coin cells assembled using carbon-coated sodium iron pyrophosphate prepared in Example 1 and the comparative example as positive electrode materials.

[0035] Figure 2 The graph shows the long-cycle performance of sodium-ion coin cells assembled using carbon-coated sodium iron phosphate pyrophosphate prepared in Example 1 and the comparative example as positive electrode materials at 20C. Detailed Implementation

[0036] To better understand the present invention, the following embodiments further illustrate the content of the present invention, but the content of the present invention is not limited to the following embodiments.

[0037] In the following examples, the iron-phosphorus slag used is the iron-phosphorus slag after lithium extraction from waste lithium iron phosphate cathode material, with a phosphorus content of 20-25%, an iron content of 30-40%, and a carbon content of ≤0.01%.

[0038] Example 1

[0039] A method for preparing carbon-coated sodium iron phosphate pyrophosphate using iron-phosphate slag includes the following steps:

[0040] 1) Add 40% nitric acid to the iron-phosphorus slag, control the mass ratio of iron-phosphorus slag to nitric acid to be 3:8, dissolve for 20 minutes and then filter to obtain an acid hydrolysate containing iron and phosphorus, and detect the concentration of iron and phosphorus ions in it.

[0041] 2) Add 30% ammonia water to the acid hydrolysate to adjust the pH of the acid hydrolysate to 5, then add ammonium oxalate. The molar ratio of ammonium oxalate to iron ions in the acid hydrolysate is 2:1. Heat to 30°C and stir for 50 minutes to obtain reaction solution A.

[0042] 3) Add ammonium dihydrogen phosphate to reaction solution A to adjust the molar ratio of iron to phosphorus in reaction solution A to 1:1.5, add sodium citrate to adjust the molar ratio of iron to sodium in reaction solution A to 1:1.4, and then add polydopamine and glucose in a mass ratio of 1:1 as carbon sources. The amount of carbon source added is 30% of the total mass of ammonium dihydrogen phosphate and sodium citrate. After stirring and dissolving, grind into a uniform slurry.

[0043] 4) The uniform slurry obtained in step 3) is spray-dried at an inlet air temperature of 240°C and an outlet air temperature of 90°C to obtain the precursor.

[0044] 5) The precursor was placed in a high-temperature furnace filled with nitrogen atmosphere and sintered at 600℃ for 12h. After crushing, carbon-coated iron pyrophosphate with a particle size of 0.5-20μm was obtained.

[0045] Example 2

[0046] A method for preparing carbon-coated sodium iron phosphate pyrophosphate using iron-phosphate slag includes the following steps:

[0047] 1) Add 30% nitric acid to the iron-phosphorus slag, control the mass ratio of iron-phosphorus slag to nitric acid to be 2:7, dissolve for 20 minutes and then filter to obtain an acid hydrolysate containing iron and phosphorus, and detect the concentration of iron and phosphorus ions in it.

[0048] 2) Add 25% ammonia water to the acid hydrolysate to adjust the pH of the acid hydrolysate to 6, then add ammonium oxalate. The molar ratio of ammonium oxalate to iron ions in the acid hydrolysate is 1.5:1. Heat to 40°C and stir for 30 minutes to obtain reaction solution A.

[0049] 3) Add ammonium dihydrogen phosphate to reaction solution A to adjust the molar ratio of iron to phosphorus in reaction solution A to 1:1.4, add sodium citrate to adjust the molar ratio of iron to sodium in reaction solution A to 1:1.35, and then add polydopamine and glucose in a mass ratio of 1:2 as carbon sources. The amount of carbon source added is 40% of the total mass of ammonium dihydrogen phosphate and sodium citrate. After stirring and dissolving, grind into a uniform slurry.

[0050] 4) The uniform slurry obtained in step 3) is spray-dried at an inlet air temperature of 260°C and an outlet air temperature of 100°C to obtain the precursor.

[0051] 5) The precursor was placed in a high-temperature furnace filled with nitrogen atmosphere and sintered at 800℃ for 12h. After crushing, carbon-coated iron phosphate pyrophosphate with a particle size of 0.5~20μm was obtained.

[0052] Comparative Example

[0053] The other steps of the comparative example are the same as those in Example 1, except that: only glucose is used as the carbon source instead of polydopamine and glucose.

[0054] The carbon-coated sodium iron pyrophosphate prepared in Example 1 and the comparative example were used as positive electrode materials to prepare sodium-ion batteries. The specific method was as follows: the carbon-coated sodium iron pyrophosphate positive electrode material was added to a mortar with acetylene black and polyvinylidene fluoride at a mass ratio of 7:2:1 and ground for 20 minutes to make it uniform; then an appropriate amount of N-methylpyrrolidone (NMP) was added and ground to make a uniform slurry. The obtained slurry was then coated on the surface of aluminum foil and dried to obtain the working electrode, which was then formed into a circular electrode sheet with a diameter of 12 mm by a stamping machine; the sodium metal sheet was used as the counter electrode, and a circular electrode sheet with a diameter of 14 mm was made as the negative electrode; coin cells were assembled in an argon-protected glove box. The tested battery model was a 2032 coin cell. The separator was a glass fiber separator. The electrolyte was 1M NaClO4 dissolved in a mixed solvent of ethylene carbonate (EC) and propylene carbonate (PC) with a volume ratio of 1:1, and 5% fluoroethylene carbonate (FEC) was added as an additive. The assembled battery underwent electrochemical performance testing, and the results are shown below. Figure 1 and Figure 2 As shown.

[0055] Figure 1 The graph shows the rate performance of sodium-ion coin cells assembled using carbon-coated sodium iron phosphate pyrophosphate prepared in Example 1 and the comparative example as the positive electrode material. Figure 1 As shown, at current densities of 0.1, 0.2, 0.5, 1, 5, 10, 20, and 30C, the reversible discharge specific capacities of the nitrogen-doped carbon-coated sodium iron phosphate pyrophosphate cathode material prepared in Example 1 were 112.40, 111.59, 110.40, 109.18, 104.62, 101.74, 93.51, and 88.30 mAh / g, respectively, all significantly higher than those of the carbon-coated sodium iron phosphate pyrophosphate cathode material prepared in the comparative example. This indicates that the defect structure introduced by the nitrogen-doped carbon layer accelerates electron conduction and improves the rate performance of the material. When the current density recovers from 30C to 0.2C, the discharge specific capacity of the cathode material prepared in Example 1 remains at 111.14 mAh / g, only lower than the initial value of 0.45 mAh / g, indicating that the nitrogen-doped carbon-coated sodium iron phosphate pyrophosphate cathode material possesses excellent structural reversibility.

[0056] Figure 2The graph shows the long-term cycling performance of sodium-ion coin cells assembled using carbon-coated sodium iron pyrophosphate (SO4) prepared in Example 1 and the comparative example as cathode materials at 20C. The initial discharge specific capacities of Example 1 and the comparative example are 92.31 mAh / g and 82.67 mAh / g, respectively. After 300 cycles, the discharge specific capacity of Example 1 remains at 92.13 mAh / g, showing almost no capacity decay compared to the initial cycle, with a capacity retention of 99.8%, significantly higher than the 94.6% capacity retention of the comparative example.

[0057] The above embodiments are merely examples for clear illustration and are not intended to limit the implementation. Those skilled in the art will recognize that other variations or modifications can be made based on the above description. It is neither necessary nor possible to exhaustively list all possible implementations, and any obvious variations or modifications derived therefrom are still within the scope of protection of this invention.

Claims

1. A method for preparing carbon-coated sodium iron phosphate with iron phosphorus slag, characterized in that, Includes the following steps: 1) Add nitric acid to the iron-phosphorus slag to dissolve it, filter it, and obtain an acid hydrolysate containing iron and phosphorus; 2) Add ammonia to the acid hydrolysate to adjust the pH to 5-6, then add ammonium oxalate. The molar ratio of ammonium oxalate to iron ions in the acid hydrolysate is (1.5-2):

1. Heat to 30-40℃ and stir for 30-50 minutes to obtain reaction solution A. 3) Add phosphorus source and sodium source to reaction solution A, then add polydopamine and glucose in a mass ratio of 1:(1~2) as carbon source, stir to dissolve, and grind into a uniform slurry; 4) The homogeneous slurry obtained in step 3) is spray-dried to obtain the precursor; 5) The precursor was sintered at high temperature in a nitrogen atmosphere to obtain carbon-coated sodium iron pyrophosphate.

2. The method for preparing carbon-coated sodium iron phosphate according to claim 1, characterized in that, The iron-phosphorus slag is the iron-phosphorus slag after lithium extraction from waste lithium iron phosphate cathode material, with a phosphorus content of not less than 20%, an iron content of not less than 30%, and a carbon content of not more than 0.01%.

3. The method for preparing carbon-coated sodium iron phosphate pyrophosphate using iron-phosphorus slag according to claim 1, characterized in that, The mass concentration of the nitric acid is 30-40%; the mass ratio of the iron-phosphorus slag to the nitric acid is (2-3):(7-8).

4. The method for preparing carbon-coated sodium iron phosphate pyrophosphate using iron-phosphorus slag according to claim 1, characterized in that, The mass concentration of the ammonia water is 25-30%.

5. The method for preparing carbon-coated sodium iron phosphate pyrophosphate using iron-phosphorus slag according to claim 1, characterized in that, The phosphorus source is ammonium dihydrogen phosphate; the phosphorus source adjusts the molar ratio of iron to phosphorus in reaction solution A to 1:(1.4~1.5).

6. The method for preparing carbon-coated sodium iron phosphate pyrophosphate using iron-phosphorus slag according to claim 1, characterized in that, The sodium source is at least one of sodium acetate, sodium carbonate, and sodium citrate; the sodium source adjusts the molar ratio of iron to sodium in reaction solution A to 1:(1.35~1.4).

7. The method for preparing carbon-coated sodium iron phosphate pyrophosphate using iron-phosphorus slag according to claim 1, characterized in that, The amount of carbon source added is 30-40% of the total mass of phosphorus and sodium sources.

8. The method for preparing carbon-coated sodium iron phosphate pyrophosphate using iron-phosphorus slag according to claim 1, characterized in that, The inlet air temperature of the spray dryer is 240~260℃, and the outlet air temperature is 90~100℃.

9. The method for preparing carbon-coated sodium iron phosphate pyrophosphate using iron-phosphorus slag according to claim 1, characterized in that, The high-temperature sintering process involves a sintering temperature of 600-800℃ and a sintering time of 10-12 hours.

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