A nitrogen-phosphorus co-doped carbon-coated cathode material of NaFePO4F, its preparation method and application

Abstract

This invention relates to the field of battery technology, and in particular to a nitrogen-phosphorus co-doped carbon-coated cathode material of NaFePO4F, its preparation method, and its application. The preparation method of the NaFePO4F nitrogen-phosphorus co-doped carbon-coated cathode material includes the following steps: mixing and reacting a sodium source, an iron source, and a phosphorus source to prepare a NaFePO4F precursor; mixing the NaFePO4F precursor, rice husk powder, bamboo leaf powder, and melamine, ball milling, and drying to obtain a precursor mixture; carbonizing the precursor mixture under a programmed temperature rise under inert gas protection; and etching, washing, and drying the carbonized mixture with hydrofluoric acid to obtain the NaFePO4F nitrogen-phosphorus co-doped carbon-coated cathode material. This invention introduces multi-element biomass and nitrogen and phosphorus element mixing, optimizes the pore structure and interface of the carbon coating layer on the NaFePO4F surface, and solves the core contradiction problem of the incompatibility between "electron conduction" and "ion transport" in traditional carbon coating.

Description

Technical Field

[0001] This invention relates to the field of battery technology, and in particular to a nitrogen-phosphorus co-doped carbon-coated cathode material of NaFePO4F, its preparation method, and its application. Background Technology

[0002] Among sodium-ion battery cathode materials, iron-based polyanionic materials, especially sodium iron fluorophosphate (NaFePO4F), have attracted much attention due to their unique advantages: they are composed of inexpensive and readily available elements such as iron, phosphorus, oxygen, and fluorine, resulting in extremely low cost; their robust polyanionic framework endows the material with ultra-long cycle life and excellent thermal stability, as well as high safety. However, the commercial application of NaFePO4F is severely limited by its inherently low intrinsic electronic conductivity. This characteristic leads to sluggish electrode reaction kinetics and low utilization of active materials, resulting in poor rate performance and low volumetric energy density of the battery.

[0003] To address the low electrical conductivity of NaFePO4F, carbon coating is currently the most common and effective technique. By constructing a conductive network on the surface of active material particles, its electronic conductivity is enhanced.

[0004] Patent application CN121506892A discloses a cathode material and a sodium-ion battery cathode sheet. The cathode material includes a mixed sodium iron phosphate matrix and a dual-carbon heterostructure penetrating the interior and exterior of the mixed sodium iron phosphate matrix. The general formula of the mixed sodium iron phosphate matrix is ​​Na4Fe3(PO4)2P2O7. The dual-carbon heterostructure includes a conductive carbon material and pyrolysis products of an organic carbon source precursor. The gas generated by the pyrolysis of the organic carbon source precursor foams the mixed sodium iron phosphate matrix to form a three-dimensional porous structure. The pyrolysis products of the organic carbon source precursor coat the surface of the mixed sodium iron phosphate matrix particles. The conductive carbon material penetrates the three-dimensional porous structure and connects the mixed sodium iron phosphate matrix particles coated by the pyrolysis products of the organic carbon source precursor, thereby forming a three-dimensional porous interconnected conductive network. Patent application CN119230791A discloses a sodium iron pyrophosphate cathode material, including sodium iron pyrophosphate and a coating layer located on the outer surface of the sodium iron pyrophosphate. The coating layer includes conductive carbon and SiO2. The SiO2 in the coating layer can separate sodium iron pyrophosphate, thus refining the material size. The conductive carbon in the coating layer can enhance conductivity and improve rate performance.

[0005] Existing traditional carbon coating technologies have several limitations. First, there is a contradiction of functional limitation: while the dense, amorphous carbon layer formed after the carbonization of conventional carbon sources improves electronic conductivity, it also hinders sodium ion transport and covers material pores, increasing ion migration resistance and failing to achieve coordinated electron and ion transport, thus limiting rate performance. Second, structural stability is insufficient; a single carbon coating layer cannot effectively buffer the micro-volume changes of the active material during long-term cycling, making it prone to cracking or peeling, leading to the loss of connection between some active materials and the conductive network, resulting in capacity decay. Third, there are cost and environmental concerns; to achieve effective coating, some solutions use expensive carbon materials such as carbon nanotubes and graphene, which is detrimental to cost control. Therefore, it is urgent to optimize the pore structure and interface of the carbon coating layer on the NaFePO4F surface, while simultaneously resolving the core contradiction of the incompatibility between electronic conduction and ion transport in traditional carbon coating. Summary of the Invention

[0006] To address the aforementioned deficiencies or improvement needs of existing technologies, this invention employs a ternary system of "rice husk-bamboo leaf-melamine" to construct an in-situ coating layer with gradient doping and multi-level pores through synergistic effects during pyrolysis. This structure successfully resolves the core contradiction in traditional carbon coating where electron conduction and ion transport are mutually exclusive, ultimately achieving the superior performance of NaFePO4F material in terms of high rate capability, long cycling duration, and high stability, laying a solid foundation for its practical application in large-scale energy storage.

[0007] To achieve the above objectives, the present invention provides the following technical solution: In a first aspect, this application provides a nitrogen-phosphorus co-doped carbon-coated cathode material of NaFePO4F, the raw materials of which include: Sodium source, iron source, phosphorus source, fluorine source, rice husk powder, bamboo leaf powder and melamine; The sodium source, iron source, phosphorus source and fluorine source contain Na:Fe:P:F in a molar ratio of 1:1:1:1; The sodium source, iron source, phosphorus source and fluorine source are mixed and reacted to obtain the NaFePO4F precursor. The mass ratio of the NaFePO4F precursor to rice husk powder to bamboo leaf powder to melamine is 100:10-15:5-10:3-5.

[0008] Furthermore, the sodium source is selected from at least one of sodium pyrophosphate, sodium carbonate, sodium nitrate, and sodium dihydrogen phosphate.

[0009] Furthermore, the iron source is selected from at least one of ferric acetate, ferric nitrate, ferrous oxalate, ferric chloride, ferric oxide, and ferric oxide.

[0010] Furthermore, the phosphorus source is selected from at least one of sodium pyrophosphate, ammonium phosphate, ammonium dihydrogen phosphate, diammonium hydrogen phosphate, and sodium dihydrogen phosphate.

[0011] Furthermore, the fluorine source is selected from at least one of sodium fluoride and ammonium bifluoride.

[0012] Secondly, this application provides a method for preparing the above-mentioned NaFePO4F nitrogen-phosphorus co-doped carbon-coated cathode material, comprising the following steps: The sodium, iron, phosphorus and fluorine sources were mixed and ball-milled to prepare the NaFePO4F precursor. NaFePO4F precursor, rice husk powder, bamboo leaf powder and melamine were mixed, ball-milled and dried to obtain precursor mixture; The precursor mixture was subjected to programmed temperature carbonization under inert gas protection. After carbonization, the material is etched, washed, and dried with hydrofluoric acid to obtain NaFePO4F nitrogen-phosphorus co-doped carbon-coated cathode material.

[0013] Furthermore, the sodium source, iron source, phosphorus source and fluorine source react according to the stoichiometric ratio of Na:Fe:P:F of 1:1:1:1. To compensate for sodium volatilization at high temperature, the sodium source is weighed in excess by 3-5% when weighing the raw materials.

[0014] Furthermore, the rice husk powder and bamboo leaf powder need to be pre-treated before use. The pre-treatment involves drying them in a forced-air drying oven at 80°C for 12 hours to completely remove moisture and then passing them through a 350-500 mesh sieve for later use.

[0015] Furthermore, the rice husk powder has a particle size of <75μm, and the bamboo leaf powder has a particle size of <50μm.

[0016] Furthermore, the conditions for ball milling the sodium source, iron source, phosphorus source and fluorine source are as follows: add an appropriate amount of anhydrous ethanol as a dispersion medium, add agate grinding balls, and ball mill at a speed of 300-400 rpm for 4-12 hours. After ball milling, dry the obtained slurry with forced air at 60-80℃.

[0017] Furthermore, the conditions for ball milling the NaFePO4F precursor, rice husk powder, bamboo leaf powder, and melamine are as follows: place the NaFePO4F precursor, rice husk powder, bamboo leaf powder, and melamine in a ball mill jar, add an appropriate amount of anhydrous ethanol as a dispersion medium (liquid-solid ratio of 2-4:1), add agate grinding balls, with a ball-to-material ratio of 8-12:1, and ball mill at a speed of 300-400 rpm for 2-6 hours.

[0018] Furthermore, after the ball milling is completed, the resulting slurry is dried by forced air at 80°C for 24 hours, then ground and passed through a 150-300 mesh sieve to obtain a uniform precursor mixture.

[0019] Further, the programmed heating carbonization includes the following steps: The precursor mixture is evenly spread in an alumina crucible, with a thickness not exceeding 1 cm, and placed in the center of a tube furnace. Inert gas is continuously introduced into the furnace tube as a protective gas. The following stepped heating program is set: the precursor mixture is heated twice. The first heating is to 250~350℃ at a heating rate of 1~2℃ / min, and the holding time is 1~2.5h. The second heating is to 600~650℃ at a heating rate of 3~5℃ / min, and the holding time is 2.5~3h. After the holding time, the sample is allowed to cool naturally to room temperature under an inert gas atmosphere.

[0020] Furthermore, the initial heating causes the melamine and biomass to decompose and cross-link initially, forming a stable structure and preventing subsequent violent reactions.

[0021] Furthermore, the inert gas is argon, and the flow rate of the argon is 100-200 sccm.

[0022] Furthermore, the etching with hydrofluoric acid involves immersing the carbonized product in a 1-2% HF solution and magnetically stirring for 2-4 hours to dissolve the residual SiO2 in the rice husk and release the macroporous structure.

[0023] Furthermore, the washing deionized water is centrifuged until the supernatant is neutral (pH≈7). Furthermore, the drying conditions after hydrofluoric acid etching are vacuum drying at 80~100℃ for 8~12h, followed by grinding of the dried sample to obtain a black powder of nitrogen-phosphorus co-doped carbon-coated cathode material of NaFePO4F.

[0024] Thirdly, this application provides an application of the above-mentioned NaFePO4F nitrogen-phosphorus co-doped carbon-coated cathode material in the field of energy storage.

[0025] Compared with the prior art, the beneficial effects of the present invention are as follows: 1. After rice husks are carbonized and acid-washed, three-dimensional interconnected macropores are left in the carbon layer. These macropores serve as "electrolyte highways," allowing electrolytes to quickly penetrate and be stored, ensuring that ions can be rapidly replenished to the reaction interface during high-rate charging and discharging. 2. Bamboo leaf powder serves as both a "fiber network and a precursor phosphorus source." After carbonization, the fibers form an interwoven mesoporous network. Simultaneously, the naturally occurring phosphorus (P) in bamboo leaves is introduced into the carbon framework, achieving phosphorus doping. The mesoporous network provides shorter and faster transport paths for ions. The introduction of P, synergistic with nitrogen generation, can further regulate the electron cloud distribution on the carbon layer surface, enhancing the adsorption capacity for sodium ions.

[0026] 3. Melamine, as a nitrogen-rich precursor, produces highly reactive nitrogen-containing groups during pyrolysis. These groups interact with biomass degradation products during carbonization, resulting in gradient doping of nitrogen. This gradient effect promotes the formation of more pyrrole nitrogen (N-5) in the inner layer near the NaFePO4F material, while graphitic nitrogen (NQ) tends to form in the outer layer.

[0027] 4. The technical solution of this application solves the core contradiction that electronic conduction and ion transport cannot be achieved simultaneously in traditional carbon coating, and finally realizes the excellent performance of NaFePO4F material with high rate, long cycle and high stability, laying a solid foundation for its practical application in the field of large-scale energy storage. Detailed Implementation

[0028] The present invention will be further described below with reference to embodiments, but the scope of protection of the present invention is not limited thereto. It should be noted that the implementation steps in the embodiments can be further adjusted according to the specific experimental environment, and the implementation steps not specified are usually conditions in conventional experiments. All compounds involved in the following embodiments are commercially available pharmaceutical products.

[0029] Example 1 Sodium carbonate, ferrous oxalate, ammonium dihydrogen phosphate, and sodium fluoride were accurately weighed according to the stoichiometric ratio Na:Fe:P:F = 1.03:1:1:1 and mixed and ball-milled to obtain the NaFePO4F precursor. Rice husk powder and bamboo leaf powder were dried in an 80℃ forced-air drying oven for 12 hours and then passed through a 400-mesh sieve for later use. The polyanionic precursor, rice husk powder, bamboo leaf powder, and melamine were then ball-milled at 350 rpm for 8 hours at a mass ratio of 100:10:5:5. After drying and passing through a 200-mesh sieve, the sieved mixture was subjected to a one-step programmed temperature carbonization under argon protection. The argon flow rate was controlled at 100 sccm, and the temperature was increased to 300°C at 2℃ / min and held for 1 hour. During the carbonization stage, the temperature was increased from 300°C to 650°C at 3℃ / min and held for 3 hours. After the holding period, the sample was allowed to cool naturally to room temperature under an argon atmosphere. After carbonization, the sample was etched with 1.5% HF and stirred in a magnetic stirrer for 3 hours. Then, it was washed with deionized water by centrifugation until the supernatant was neutral (pH≈7). The final product was vacuum dried at 100°C for 12 hours. The dried sample was then lightly ground to obtain NaFePO4F@nitrogen-phosphorus co-doped hierarchical porous carbon black powder.

[0030] Example 2 Sodium pyrophosphate, ferric acetate, ammonium phosphate, and sodium fluoride were accurately weighed according to the stoichiometric ratio Na:Fe:P:F = 1.03:1:1:1 and mixed and ball-milled to obtain the NaFePO4F precursor. Rice husk powder and bamboo leaf powder were dried in an 80℃ forced-air drying oven for 12 hours and then passed through a 400-mesh sieve for later use. The polyanionic precursor, rice husk powder, bamboo leaf powder, and melamine were then ball-milled at 350 rpm for 8 hours at a mass ratio of 100:15:10:3. After drying and passing through a 150-mesh sieve, the sieved mixture was subjected to a one-step programmed temperature carbonization under argon protection. The argon flow rate was controlled at 200 sccm, and the temperature was increased to 250°C at 2℃ / min and held for 1.5 hours. During the carbonization stage, the temperature was increased from 300°C to 650°C at 3℃ / min and held for 2.5 hours. After the holding period, the sample was allowed to cool naturally to room temperature under an argon atmosphere. After carbonization, the sample was etched with 1.0% HF and stirred in a magnetic stirrer for 4 hours. Then, it was washed with deionized water by centrifugation until the supernatant was neutral (pH≈7). The final product was vacuum dried at 80°C for 10 hours. The dried sample was then lightly ground to obtain NaFePO4F@nitrogen-phosphorus co-doped hierarchical porous carbon black powder.

[0031] Example 3 Sodium dihydrogen phosphate, ferric oxide, diammonium hydrogen phosphate, and ammonium hydrogen fluoride were accurately weighed according to the stoichiometric ratio Na:Fe:P:F = 1.03:1:1:1, mixed, and ball-milled to obtain the NaFePO4F precursor. Rice husk powder and bamboo leaf powder were dried in an 80℃ forced-air drying oven for 12 hours and then passed through a 400-mesh sieve for later use. The polyanionic precursor, rice husk powder, bamboo leaf powder, and melamine were then ball-milled at a mass ratio of 100:12:8:4 for 8 hours at 350 rpm. After drying and passing through a 300-mesh sieve, the sieved mixture was subjected to a one-step programmed temperature carbonization under argon protection. The argon flow rate was controlled at 150 sccm, and the temperature was increased to 350°C at 2℃ / min and held for 2.5 hours. During the carbonization stage, the temperature was increased from 300°C to 650°C at 3℃ / min and held for 3 hours. After the holding period, the sample was allowed to cool naturally to room temperature under an argon atmosphere. After carbonization, the sample was etched with 2.0% HF and stirred in a magnetic stirrer for 2 hours. Then, it was washed with deionized water by centrifugation until the supernatant was neutral (pH≈7). The final product was vacuum dried at 90°C for 12 hours. The dried sample was then lightly ground to obtain NaFePO4F@nitrogen-phosphorus co-doped hierarchical porous carbon black powder.

[0032] Comparative Example 1 No rice husk powder was added to the materials, and all other preparation conditions were the same as in Example 1.

[0033] Comparative Example 2 No bamboo leaf powder was added to the materials, and all other preparation conditions were the same as in Example 1.

[0034] Comparative Example 3 Na₂CO₃, FeC₂O₄·2H₂O, NH₄H₂PO₄, and NaF were accurately weighed according to the stoichiometric ratio Na:Fe:P:F = 1.03:1:1:1 and ball-milled to obtain a polyanionic precursor. The polyanionic precursor, glucose, and urea were then ball-milled at a mass ratio of 100:5:5 for 8 hours at 350 rpm. Finally, the sieved mixture was subjected to a one-step programmed temperature carbonization under argon protection. The argon flow rate was controlled at 100 sccm, and the temperature was increased to 300°C at 2°C / min and held for 1 hour. During the carbonization stage, the temperature was increased from 300°C to 650°C at 3°C / min and held for 3 hours. After the holding period, the sample was allowed to cool naturally to room temperature under an argon atmosphere. After carbonization, the sample was washed with 1.5% HF and stirred in a magnetic stirrer for 3 hours. Then, it was washed with deionized water by centrifugation until the supernatant was neutral (pH≈7). The final product was vacuum dried at 100℃ for 12 hours. The dried sample was then lightly ground to obtain NaFePO4F@nitrogen-phosphorus co-doped hierarchical porous carbon black powder.

[0035] Experimental Example The performance of the cathode materials prepared in Example 1 and Comparative Examples 1-3 was tested.

[0036] Ionic conductivity testing method: The powder is pressed into a dense disc, with blocking electrodes added to both sides. The AC impedance method is used to test at 0.1~1 MHz. The bulk resistance R is obtained from the high-frequency intercept, and the ionic conductivity is calculated using σ=L / (R·S).

[0037] Battery performance testing method: The materials prepared in Example 1 and Comparative Examples 1-3 were used as the positive electrode materials for sodium-ion batteries. A metallic sodium sheet was used as the negative electrode, and a GF / A type glass fiber membrane was used as the separator. Electrolyte was added to assemble CR2032 coin cells, which were then subjected to electrochemical performance testing after standing. The CT-2001A battery testing system was used to test various electrochemical performance parameters. The voltage range for the constant current charge-discharge test was 1.5–4.0 V.

[0038] The experimental results are shown in the table below. The material prepared in Example 1 has a higher ionic conductivity than that of Comparative Examples 1-3. The specific capacity at 1C rate discharge, the capacity retention rate after 200 cycles at 1C rate, the capacity retention rate after 4C rate discharge, and the capacity retention rate after 100 cycles at 4C rate are all significantly higher than those of Comparative Examples 1-3.

[0039]

Claims

1. A nitrogen-phosphorus co-doped carbon-coated cathode material of NaFePO4F, characterized in that, Raw materials include: Sodium source, iron source, phosphorus source, fluorine source, rice husk powder, bamboo leaf powder and melamine; The sodium source, iron source, phosphorus source and fluorine source contain Na:Fe:P:F in a molar ratio of 1:1:1:1; The sodium source, iron source, phosphorus source and fluorine source are mixed and reacted to obtain the NaFePO4F precursor. The mass ratio of the NaFePO4F precursor to rice husk powder to bamboo leaf powder to melamine is 100:10-15:5-10:3-5.

2. The cathode material according to claim 1, characterized in that, The sodium source is selected from at least one of sodium pyrophosphate, sodium carbonate, sodium nitrate, and sodium dihydrogen phosphate.

3. The cathode material according to claim 1 or 2, characterized in that, The iron source is selected from at least one of ferric acetate, ferric nitrate, ferrous oxalate, ferric chloride, ferric oxide, and ferric oxide.

4. The cathode material according to claim 1 or 2, characterized in that, The phosphorus source is selected from at least one of sodium pyrophosphate, ammonium phosphate, ammonium dihydrogen phosphate, diammonium hydrogen phosphate, and sodium dihydrogen phosphate.

5. The cathode material according to claim 1 or 2, characterized in that, The fluorine source is selected from at least one of sodium fluoride and ammonium bifluoride.

6. A method for preparing a nitrogen-phosphorus co-doped carbon-coated cathode material of NaFePO4F as described in any one of claims 1-5, characterized in that, Including the following steps: Sodium, iron, phosphorus and fluorine sources were mixed and ball-milled to obtain NaFePO4F precursor; NaFePO4F precursor, rice husk powder, bamboo leaf powder and melamine were mixed and ball-milled to obtain a precursor mixture; The precursor mixture was subjected to programmed temperature carbonization under inert gas protection. After carbonization, the material is etched, washed, and dried with hydrofluoric acid to obtain NaFePO4F nitrogen-phosphorus co-doped carbon-coated cathode material.

7. The preparation method according to claim 6, characterized in that, The rice husk powder has a particle size of <75μm, and the bamboo leaf powder has a particle size of <50μm.

8. The preparation method according to claim 6 or 7, characterized in that, The programmed heating carbonization includes the following steps: heating the precursor mixture twice, first heating to 250~350℃ at a heating rate of 1~2℃ / min and holding for 1~2.5h, and second heating to 600~650℃ at a heating rate of 3~5℃ / min and holding for 2.5~3h.

9. The preparation method according to claim 6 or 7, characterized in that, The inert gas is argon, and the flow rate of the argon is 100-200 sccm.

10. The preparation method according to claim 6 or 7, characterized in that, The mass concentration of the hydrofluoric acid is 1-2%.

11. The application of a nitrogen-phosphorus co-doped carbon-coated cathode material of NaFePO4F as described in any one of claims 1-5 in the field of energy storage.

Images