A carbon layer covalently anchored sodium iron phosphate pyrophosphate positive electrode material, a preparation method and application thereof

Abstract

This invention relates to a carbon-coated covalently anchored sodium iron pyrophosphate cathode material, its preparation method, and its application, belonging to the field of sodium-ion battery material technology. It solves at least one of the problems of weak interfacial bonding and easy carbon layer detachment during cycling in traditional carbon coating processes. The preparation method includes: S1, preparing a carbon source containing pyrophosphate groups; S2, weighing sodium, iron, and phosphorus sources, adding the carbon source, dispersant, and solvent obtained in S1, and ball milling to obtain a precursor slurry; drying the precursor slurry to obtain a precursor powder; S3, pre-pyrolyzing the precursor powder from S2 to obtain a carbon-coated pre-anchored precursor; S4, directly heating the carbon-coated pre-anchored precursor from S3 for sintering to crystallize NFPP nuclei and stabilize the carbon layer; cooling and pulverizing after calcination to obtain the carbon-coated covalently anchored sodium iron pyrophosphate cathode material.

Description

Technical Field

[0001] This invention relates to the field of sodium-ion battery materials technology, and in particular to a carbon-covalently anchored sodium iron pyrophosphate cathode material, its preparation method, and its application. Background Technology

[0002] Sodium-ion batteries have become a core technology for replacing lithium-ion batteries in the energy storage field due to their advantages such as abundant sodium resources, low production costs, and suitability for large-scale energy storage scenarios. Sodium iron pyrophosphate (Na4Fe3(PO4)2P2O7, NFPP), as a polyanionic sodium-ion battery cathode material, possesses characteristics such as three-dimensional ion transport channels, excellent structural stability, stable voltage platform, and environmental friendliness, making it a cathode material system with great industrialization potential.

[0003] However, NFPP materials have extremely low intrinsic electronic conductivity (≤10). -7 S·cm -1 Sodium ions diffuse slowly, requiring carbon coating modification to improve electrochemical performance. Simultaneously, pyrophosphate is easily decomposed during NFPP crystallization, readily generating impurity phases such as NaFePO4 and Fe3(PO4)2, reducing the material's crystal phase purity and electrochemical activity. Traditional carbon coating processes rely solely on physical adsorption, resulting in weak interfacial bonding between the carbon layer and the NFPP crystal nucleus. Under the volumetric stress of charge-discharge cycles, the carbon layer is prone to cracking and detachment, failing to provide long-term stable protection for the crystal nucleus and leading to a significant decrease in battery cycle life. Furthermore, most processes employ step-by-step sintering, which is cumbersome and energy-intensive.

[0004] In addition, conventional carbon-coated NFPP technologies mostly use common carbon sources such as glucose and phenolic resin, which can only achieve physical coating. They cannot solve the common industry problems such as easy carbon layer detachment, easy generation of impurity phases, weak interfacial bonding, and poor long-cycle performance, which seriously restricts the commercial application of NFPP cathode materials. Summary of the Invention

[0005] Based on the above analysis, the present invention aims to provide a carbon-coated sodium iron pyrophosphate cathode material, its preparation method, and its application, in order to solve at least one of the following problems: weak interfacial bonding of traditional carbon coating, easy detachment of carbon layer during cycling, decomposition of pyrophosphate ions during NFPP crystallization process, easy generation of impurity phases, low intrinsic electronic conductivity of NFPP, poor long-cycle performance of materials, cumbersome preparation process, and high energy consumption.

[0006] On one hand, the present invention provides a method for preparing a carbon-layer covalently anchored sodium iron pyrophosphate cathode material, comprising: S1, prepare a carbon source containing pyrophosphate groups; S2, weigh out sodium source, iron source and phosphorus source, add carbon source, dispersant and solvent obtained from S1, and ball mill to obtain precursor slurry; dry the precursor slurry to obtain precursor powder; S3, the precursor powder in S2 is pre-pyrolyzed with carbon source to obtain carbon layer pre-anchored precursor. S4 involves directly heating the carbon-anchored precursor from S3 for sintering, thereby crystallizing the NFPP nuclei and stabilizing the carbon layer. After calcination, the material is cooled and pulverized to obtain a carbon-covalently anchored sodium iron pyrophosphate cathode material.

[0007] Furthermore, the carbon source includes at least one of the following: phenolic resin-based carbon source, vinyl phosphonic acid-based carbon source, and hydroxymethyl phosphonic acid-based carbon source.

[0008] Furthermore, in S2, the amount of carbon source added is 3.5% to 7.5% of the total mass of sodium, iron, and phosphorus sources.

[0009] Furthermore, in S3, the pyrolysis temperature of the carbon source preheating is 300~400℃.

[0010] Furthermore, in S4, the sintering temperature is 580~680℃.

[0011] On the other hand, the present invention provides a carbon-covalently anchored sodium iron pyrophosphate cathode material, comprising: an NFPP crystal nucleus and a carbon layer coated on the NFPP crystal nucleus.

[0012] Furthermore, the phase purity of the NFPP nucleus is ≥99.5%, of which Fe 2+ / Fe 3+ The molar ratio is (2.95~3.0):0.1; and / or, the primary particle size of the NFPP nuclei is 100~300nm, and the NFPP nuclei are spherical.

[0013] Furthermore, the thickness of the carbon layer is 0.8~2.5 nm, the carbon content is 1.2%~2.8% of the mass of the NFPP crystal nucleus, and the phosphorus content in the carbon layer is 0.5 at%~1.8 at.

[0014] Furthermore, the electronic conductivity of the sodium iron pyrophosphate cathode material is ≥1.2×10⁻⁶. -5 S·cm -1 The capacity retention rate is ≥93.9% after 500 cycles at 1C, and the carbon layer shedding rate is ≤0.3% after 1000 cycles at 1C.

[0015] On the other hand, the present invention also provides an application of the above-mentioned sodium iron pyrophosphate cathode material or the sodium iron pyrophosphate cathode material prepared by the above preparation method in sodium-ion batteries.

[0016] This invention can achieve at least one of the following beneficial effects: 1. This invention prepares a carbon-coated sodium iron pyrophosphate cathode material by using a carbon source containing pyrophosphate groups. The carbon layer and NFPP are chemically anchored by lattice matching and stable PO-Fe covalent bonds, thereby improving the bonding force of the carbon coating interface and completely eliminating the problem of carbon layer detachment caused by physical adsorption.

[0017] 2. In this invention, the carbon source can also provide directional pyrophosphate feeding for NFPP crystallization, thereby inhibiting pyrophosphate decomposition and giving NFPP high crystal phase purity, with a crystal phase purity ≥99.5% and impurity phase content ≤0.2%, and impurity phase is almost completely suppressed.

[0018] 3. In the sodium iron pyrophosphate cathode material of the present invention, the single-layer covalently anchored carbon layer can construct a continuous conductive network, thereby increasing the electronic conductivity of the material by 1 to 2 orders of magnitude; at the same time, the carbon layer, which is doubly anchored by lattice matching and PO-Fe covalent bonds, is not easy to fall off, thus greatly optimizing the rate performance and cycle performance.

[0019] 4. This invention adopts a one-step linkage process of preheating and final calcination. The preheating step realizes carbon source preheating, lattice matching and PO-Fe covalent anchoring. Then, based on the preheating, the temperature is directly raised for sintering, and the complete crystallization of NFPP crystal nuclei and carbon layer stabilization are completed simultaneously. There is no need for step-by-step sintering or separate outer layer coating, which simplifies the process, shortens the process flow, reduces energy consumption, and is suitable for industrial mass production.

[0020] 5. The sodium iron pyrophosphate cathode material of the present invention has a crystal phase purity ≥99.5%, an impurity phase content ≤0.2%, and an electronic conductivity ≥1.2×10⁻⁶. -5 S·cm -1 0.1C discharge capacity ≥107.8mAh·g -1 5C discharge capacity ≥ 91.5 mAh·g -1 The capacity retention rate is ≥93.9% after 500 cycles at 1C, and the carbon layer shedding rate is ≤0.3% after 1000 cycles at 1C.

[0021] In this invention, the above-described technical solutions can be combined with each other to achieve more preferred combinations. Other features and advantages of this invention will be set forth in the following description, and some advantages may become apparent from the description or be learned by practicing the invention. The objects and other advantages of this invention can be realized and obtained from what is particularly pointed out in the description and drawings. Attached Figure Description

[0022] The accompanying drawings are for illustrative purposes only and are not intended to limit the invention. Throughout the drawings, the same reference numerals denote the same parts.

[0023] Figure 1 This is an electron microscope image of the NFPP cathode material prepared according to Example 3 of the present invention.

[0024] Figure 2 This is an electron microscope image of the carbon layer in the NFPP cathode material prepared according to Example 3 of the present invention. Detailed Implementation

[0025] To make the objectives, technical solutions, and advantages of the present invention clearer, exemplary embodiments of the present invention will be described below in conjunction with the accompanying drawings. It should be understood that the specific embodiments described herein are merely illustrative of the invention and are not intended to limit the invention. For clarity and brevity, not all features of actual embodiments are described in the specification.

[0026] Embodiments of the present invention provide a method for preparing a carbon-layer covalently anchored sodium iron pyrophosphate cathode material, specifically including the following steps: S1, prepare a carbon source containing pyrophosphate groups; S2, weigh out sodium source, iron source and phosphorus source, add carbon source, dispersant and solvent obtained from S1, and ball mill to obtain precursor slurry; dry the precursor slurry to obtain precursor powder; S3, the precursor powder in S2 is pre-pyrolyzed with carbon source to obtain carbon layer pre-anchored precursor. S4 involves directly heating the carbon-anchored precursor from S3 for sintering, thereby crystallizing the NFPP nuclei and stabilizing the carbon layer. After calcination, the material is cooled and pulverized to obtain a carbon-covalently anchored sodium iron pyrophosphate cathode material.

[0027] In embodiments of this invention, a carbon source containing pyrophosphate groups is used, which can form a covalently anchored carbon layer coating the NFPP crystal nucleus, improving the interfacial bonding force and ensuring the stable coating of the carbon layer on the NFPP crystal nucleus. The carbon layer and NFPP are chemically anchored through lattice matching and the formation of stable PO-Fe covalent bonds, thereby enhancing the interfacial bonding force and completely eliminating the problem of carbon layer detachment caused by physical adsorption. Specifically, a large number of pyrophosphate groups (P2O7) are exposed on the surface of the NFPP crystal. 4- The crystal plane contains coordinatingly unsaturated Fe atoms. 2+ Active sites; pyrophosphate units in the carbon layer and P2O7 on the NFPP crystal plane 4- With identical bond lengths, bond angles, and spatial configurations, lattice orientation matching can be achieved; simultaneously, during pyrolysis, the electron-rich oxygen atoms at the ends of the P2O7 groups in the carbon layer interact with the Fe atoms on the NFPP surface. 2+ A coordination reaction occurs, which can form a stable PO-Fe covalent bond.

[0028] The embodiments of the present invention employ a one-step linkage process of prepyrolysis and final calcination. Prepyrolysis achieves carbon source prepyrolysis, lattice matching, and PO-Fe covalent anchoring. Then, based on the prepyrolysis, the temperature is directly raised for sintering, simultaneously completing the complete crystallization of NFPP nuclei and the stabilization of the carbon layer. This eliminates the need for step-by-step sintering and separate outer layer coating, simplifying the process flow by 40% and reducing energy consumption by more than 35%, making it suitable for industrial mass production.

[0029] According to some embodiments of the present invention, the carbon source includes at least one of phenolic resin-based carbon source, vinyl phosphonic acid-based carbon source, and hydroxymethylphosphonic acid-based carbon source. The carbon source of the present invention is obtained by reacting at least one of phenolic resin, vinyl phosphonic acid, and hydroxymethylphosphonic acid with pyrophosphate.

[0030] When using a phenolic resin-based carbon source, step S1, preparing the carbon source, specifically includes the following steps: S11, mix thermoplastic phenolic resin with solvent, and heat and stir under nitrogen protection until the phenolic resin is completely dissolved; S12, add a mixture of pyrophosphate and catalyst dropwise to the phenolic resin solution obtained in step S11. After the addition is complete, heat the solution under reflux to react. During the reaction, the generated water is continuously removed to promote the formation of POC bonds. S13, after the reaction is completed, cool to room temperature, pour in deionized water, and a pale yellow solid precipitates. Filter, wash and vacuum dry to obtain a phenolic resin-based carbon source.

[0031] Specifically, in S11, the thermoplastic phenolic resin used has a weight-average molecular weight (Mw) of 500-1000 and a softening point of 80-90℃. The solvent is a mixture of anhydrous ethanol and dimethylformamide, with a volume ratio of anhydrous ethanol to dimethylformamide of (6.5-7.5):(2.5-3.5), for example, 6.5:2.5, 7:3, 7.5:3.5, etc.

[0032] In S11, the mass ratio of phenolic resin to solvent volume is (1.5~1.7) g : (7~9) mL, for example, 1.5 g : 7 mL, 1.6 g : 8 mL, 1.7 g : 9 mL. The heating temperature is 50~65℃, for example, 50℃, 55℃, 60℃, 65℃, etc.; the stirring time is 25~35 min, for example, 25 min, 30 min, 35 min.

[0033] In S12, the molecular skeleton of thermoplastic phenolic resin has a large number of phenolic hydroxyl groups -OH. The molar ratio of PF-OH to pyrophosphate is 1:0.5~0.6, for example, 1:0.5, 1:0.55, 1:0.6. Here, PF-OH represents the phenolic hydroxyl functional group in the thermoplastic phenolic resin molecule that can participate in the esterification reaction, that is, the amount of phenolic hydroxyl substance.

[0034] In S12, the catalyst is p-toluenesulfonic acid, and the amount added is 1.5wt% to 2.5wt% of the phenolic resin mass, for example, 1.5wt%, 2wt%, or 2.5wt%. The mixture of pyrophosphoric acid and catalyst is added dropwise over 8 to 12 minutes; the reflux temperature is 80 to 90°C, for example, 80°C, 82°C, 85°C, 88°C, or 90°C; and the reflux time is 3 to 5 hours, for example, 3 hours, 3.5 hours, 4 hours, 4.5 hours, or 5 hours.

[0035] In S13, the vacuum drying temperature is 55~65℃, and the drying time is 10~14h.

[0036] Phenolic resin (PF) contains a large number of -OH groups, which undergo dehydration esterification with pyrophosphate (H4P2O7) to form POC covalent bonds. This process also preserves the pyrophosphate (-O-P2O6-) active sites in the prepared carbon source, which are then used for subsequent bonding with Fe on the NFPP surface. 3+ / Fe 2+ PO-Fe anchoring bonds are formed.

[0037] When using a vinylphosphonic acid-based carbon source, step S1 for preparing the carbon source specifically includes the following steps: S11, Vinylphosphonic acid is mixed with a solvent under an inert atmosphere and stirred in an ice bath; the solvent may be tetrahydrofuran. S12, slowly add the dehydrating agent dicyclohexylcarbodiimide (DCC) to the solution obtained in step S11, and then add pyrophosphate dropwise; after the addition is complete, stir the reaction at room temperature to generate a white DCC-urea precipitate; S13, filter to remove precipitate, concentrate the filtrate by rotary evaporation to obtain a pale yellow viscous liquid; recrystallize with ethyl acetate / n-hexane, and then dry under vacuum to obtain a vinylphosphonic acid-based carbon source.

[0038] In S11, the solvent can be tetrahydrofuran, and the mass ratio of vinylphosphonic acid to the volume of the solvent is (6~7) g: (45~55) mL; the ice bath stirring temperature is 0~5℃, and the stirring time is 10~20 min.

[0039] In S12, the molar ratio of pyrophosphate to vinylphosphonic acid is (0.7~0.9):1, for example, 0.7:1, 0.8:1, or 0.9:1. The amount of dicyclohexylcarbodiimide added is 13mol%~17mol% of vinylphosphonic acid, for example, 13mol%, 14mol%, 15mol%, 16mol%, or 17mol%. The pyrophosphate is added dropwise over 12~18 minutes. The reaction time is 5~7 hours, for example, 5 hours, 6 hours, or 7 hours.

[0040] In S13, the rotary evaporation temperature is 35~45℃ and the pressure is -0.8MPa. The vacuum drying temperature is 35~45℃ and the drying time is 7~9h.

[0041] Specifically, vinylphosphonic acid (VPA, CH2=CH-PO3H2) contains -PO3H2, which undergoes dehydration condensation with pyrophosphate to form a pyrophosphate monoester, thus attaching a pyrophosphate unit to the end of the vinylphosphonic acid molecule. During prepyrolysis, the pyrophosphate group in the carbon source is activated, and its terminal electron-rich oxygen atom coordinates with the unsaturated Fe on the NFPP surface. 2+ A coordination reaction occurs, forming stable PO-Fe covalent bonds, achieving atomic-level chemical anchoring between the carbon layer and the crystal nucleus.

[0042] When using a hydroxymethylphosphonic acid-based carbon source, step S1 for preparing the carbon source specifically includes the following steps: S11, mix hydroxymethylphosphonic acid with a solvent, and heat and stir under nitrogen protection until clear; S12, add pyrophosphoric acid and concentrated sulfuric acid dropwise to the solution obtained in step S11, heat to the first temperature to carry out a dehydration reaction, and evaporate the water in the system; then continue to heat to the second temperature and keep warm to strengthen the formation of POP bonds. S13: Cool the solution from step S12 to a third temperature, pour in isopropanol, and a white powder precipitates out; filter, wash with isopropanol, and dry to obtain a hydroxymethylphosphonic acid carbon source.

[0043] In S11, the solvent is a mixture of deionized water and ethylene glycol, with a volume ratio of deionized water to ethylene glycol of (0.9–1.1):(0.9–1.1); the mass ratio of hydroxymethylphosphonic acid to solvent volume is (5.3–6.3) g:(25–35 mL). The heating temperature is 65–75 °C, and the time is 15–25 min.

[0044] In S12, the molar ratio of hydroxymethylphosphonic acid to pyrophosphate is 1:(1.0~1.2), for example, 1:1, 1:1.1, 1:1.2. The amount of concentrated sulfuric acid added is 0.8wt%-1.2wt% of the total mass of pure hydroxymethylphosphonic acid and pure pyrophosphate, for example, 0.8wt%, 0.9wt%, 1wt%, 1.1wt%, 1.2wt%. The first temperature is 110~130℃ (e.g., 110℃, 120℃, 130℃), and the dehydration reaction time is 2.5~3.5h. The second temperature is 140~160℃ (e.g., 140℃, 150℃, 160℃), and the temperature is maintained for 0.8~1.2h.

[0045] In S13, the third temperature is 70~90℃ (e.g., 70℃, 80℃, 90℃). The vacuum drying temperature is 70~90℃, and the drying time is 9~10h.

[0046] Specifically, hydroxymethylphosphonic acid (HMPA, HO-CH2-PO3H2) contains -OH and -PO3H2, which undergo two-site dehydration with pyrophosphate: -OH forms POC with H4P2O7, and -PO3H2 forms POP (pyrophosphate bond) with H4P2O7; this results in the carbon source having a high density of pyrophosphate anchor sites, which strengthens its binding with the NFPP interface.

[0047] According to some embodiments of the present invention, in S2, the amount of carbon source added is 3.5% to 7.5% of the total mass of sodium, iron, and phosphorus sources, for example, 3.5%, 4%, 4.5%, 5%, 5.5%, 6%, 6.5%, 7%, and 7.5%. If the amount of carbon source added is too low, it will lead to insufficient continuity of the carbon layer, decreased lattice matching, or insufficient covalent anchoring and insufficient covalent bond density, resulting in a decrease in the interfacial bonding force between the carbon layer and the NFPP crystal nucleus and the electronic conductivity of the material, making the carbon layer prone to detachment and reducing the electrical properties of the material. If the amount of carbon source added is too high, it will lead to excessive carbon layer thickness and decreased carbon layer density. On the one hand, it will increase the proportion of inactive substances in the material, reducing the compaction density and volumetric energy density of the material; on the other hand, excessive carbon source is prone to agglomeration during pyrolysis, forming an amorphous carbon coating layer that hinders sodium ion diffusion. At the same time, pyrophosphate in excessive carbon source may cause local phosphorus segregation, affecting the complete formation of the NFPP crystal phase, resulting in a decrease in the rate performance and cycle stability of the material.

[0048] According to some embodiments of the present invention, in step S2, sodium source, iron source and phosphorus source are weighed according to the molar ratio Na:Fe:P = 4:(2.95~3.05):(4.0~4.2).

[0049] The sodium source includes one or more of the following: sodium carbonate, sodium bicarbonate, sodium acetate, sodium oxalate, sodium hydroxide, and sodium phosphate.

[0050] Iron sources include one or more of the following: ferrous oxalate, ferrous sulfate, ferrous chloride, and ferrous phosphate.

[0051] Phosphorus sources include one or more of the following: ammonium dihydrogen phosphate, diammonium hydrogen phosphate, sodium pyrophosphate, phosphoric acid, and pyrophosphate.

[0052] According to some embodiments of the present invention, in step S2, the dispersant includes at least one of polyvinylpyrrolidone (PVP) and polyethylene glycol (PEG-6000). The amount of dispersant added is 0.3% to 0.8% of the total mass of sodium, iron, and phosphorus sources, for example, 0.3%, 0.4%, 0.5%, 0.6%, 0.7%, and 0.8%. The solvent is water, and the amount of water added is 120% to 220% of the total mass of sodium, iron, and phosphorus sources, for example, 120%, 140%, 150%, 160%, 180%, 200%, and 220%. If the water content is too low, the slurry viscosity will be too high, resulting in uneven ball milling, carbon source agglomeration, poor raw material mixing, large particle size differences in NFPP precursors, and easy generation of impurity phases during subsequent crystallization. If the amount of water added is too high, the solid content of the slurry will be too low, the drying time will be prolonged, the drying energy consumption will increase significantly, and the dried powder will be hollow and porous, resulting in decreased compaction density and poor mass production efficiency.

[0053] According to some embodiments of the present invention, in step S2, the ball milling time is 1.5 to 3 hours, for example, 1.5 hours, 2 hours, 2.5 hours, or 3 hours, so as to obtain a uniform precursor slurry.

[0054] According to some embodiments of the present invention, in step S2, when drying the precursor slurry, the precursor slurry can be dried at 80~110°C for 8~12 hours. For example, the drying temperatures are 80°C, 90°C, 100°C, and 110°C, and the drying times are 8 hours, 9 hours, 10 hours, 11 hours, and 12 hours, respectively. After drying, the slurry is pulverized and then sieved through a 200~300 mesh sieve to obtain dried precursor powder.

[0055] According to some embodiments of the present invention, in S3, the pyrolysis temperature of the carbon source pre-pyrolysis is 300~400℃, and the holding time is 2~4h. For example, the pyrolysis temperatures are 300℃, 320℃, 340℃, 350℃, 360℃, 380℃, and 400℃; the holding times are 2h, 2.5h, 3h, 3.5h, and 4h. The pre-pyrolysis within this temperature range in the embodiments of the present invention enables the pre-pyrolysis of the carbon source, lattice matching, and covalent anchoring of PO-Fe, resulting in a carbon layer pre-anchored precursor.

[0056] If the preheating temperature is too low (below 300℃), the carbon source will not pyrolyze sufficiently, and the pyrophosphate groups will not be effectively activated, making it difficult to form PO-Fe covalent bonds. Simultaneously, the carbonization rate of the carbon source will be low, resulting in poor carbon layer continuity and an inability to form a complete conductive network. This leads to a decrease in interfacial bonding and electronic conductivity, making the carbon layer prone to detachment during cycling and reducing the material's electrical properties. If the preheating temperature is too high (above 400℃), the carbon source will undergo excessive pyrolysis, and the pyrophosphate groups will decompose, destroying the active sites that match the NFPP crystal planes, leading to lattice mismatch. Furthermore, excessively high temperatures can induce localized crystallization of the precursor, affecting the complete formation of subsequent NFPP and even generating impurity phases, reducing crystal phase purity, and decreasing the material's cycling stability and rate performance.

[0057] If the holding time is too short (less than 2 hours), the carbon source pyrolysis reaction will be insufficient, the pyrophosphate groups in the carbon source will not be fully activated, the PO-Fe covalent bonds will not form sufficiently, and the carbon layer will not be firmly anchored to the crystal nucleus. At the same time, the incomplete nucleation of the carbon layer will easily lead to local defects, resulting in weak interfacial bonding, poor carbon layer continuity, and a shortened material cycle life. If the holding time is too long (more than 4 hours), it will lead to over-carbonization of the carbon source, excessively high carbon layer density, and hinder the subsequent transport of sodium ions. At the same time, the excessively high temperature environment will exacerbate the decomposition of pyrophosphate in the precursor, affecting the subsequent crystallization of NFPP, and may even cause abnormal iron ion valence states, reducing the crystal phase purity and electrochemical activity of the material.

[0058] According to some embodiments of the present invention, in S4, the sintering temperature is 580~680℃, and the holding time is 5~9h. For example, the sintering temperatures are 580℃, 600℃, 620℃, 640℃, 650℃, 660℃, and 680℃, and the holding times are 5h, 6h, 7h, 8h, and 9h, respectively. After prepyrolysis, the carbon layer pre-anchored precursor can be directly heated to 580~680℃ for sintering. By performing sintering within the above temperature range, embodiments of the present invention can simultaneously achieve complete crystallization of NFPP nuclei and stabilization of the carbon layer.

[0059] If the sintering temperature is too low (below 580℃), the crystallization driving force of the NFPP nucleus is insufficient, resulting in incomplete crystal growth and the formation of amorphous or low-crystallinity structures. This leads to incomplete sodium ion diffusion channels and a decrease in rate performance. Simultaneously, the carbon layer cannot be fully graphitized, resulting in low electronic conductivity and insufficient overall electrochemical activity of the material. If the sintering temperature is too high (above 680℃), it will cause the NFPP crystal phase to decompose, generating impurity phases such as NaFePO4 and Fe3(PO4)2, significantly reducing crystal phase purity. Furthermore, excessive crystal growth and severe particle agglomeration form blind pores or closed channels, hindering electrolyte wetting and sodium ion transport. Additionally, excessively high temperatures will destroy the formed PO-Fe covalent bonds, leading to a decrease in the bonding force between the carbon layer and the nucleus interface. During cycling, the carbon layer is prone to detachment, deteriorating the long-term cycling stability of the material.

[0060] If the holding time is too short (less than 5 hours), the NFPP crystallization reaction will be insufficient, resulting in incomplete crystal development, low crystal phase purity, and poor structural stability. Simultaneously, the carbon layer stabilization process will be incomplete, leading to a loose and discontinuous carbon layer structure, low electronic conductivity, and a tendency for structural collapse during material cycling, resulting in rapid capacity decay. If the holding time is too long (more than 9 hours), it will cause excessive NFPP crystal growth, resulting in excessively large grain sizes, longer sodium ion diffusion paths, and decreased rate performance. Furthermore, prolonged high-temperature environments will exacerbate the graphitization of the carbon layer, leading to excessively high carbon layer density and hindering electrolyte wetting. In addition, prolonged sintering will promote the formation of grain boundary impurities, reducing the material's crystal phase purity and cycling stability.

[0061] According to some embodiments of the present invention, in steps S3 and S4, both prepyrolysis and sintering are carried out under an inert atmosphere. Specifically, the inert atmosphere is one or more mixed gases selected from nitrogen and argon, and the gas flow rate is 50~120 mL / min.

[0062] According to some embodiments of the present invention, in step S4, after sintering, the material is cooled to room temperature, pulverized, and passed through a 300-400 mesh sieve to obtain positive electrode material powder.

[0063] Embodiments of the present invention also provide a carbon-covalently anchored sodium iron pyrophosphate cathode material, comprising: an NFPP crystal nucleus and a carbon layer coated on the NFPP crystal nucleus.

[0064] In embodiments of the present invention, the carbon layer is covalently anchored to coat the NFPP crystal nucleus, thereby enhancing the interfacial bonding force and ensuring the stable coating of the carbon layer onto the NFPP crystal nucleus. The carbon layer and NFPP are chemically anchored through lattice matching and the formation of stable PO-Fe covalent bonds, thus improving the interfacial bonding force and completely eliminating the problem of carbon layer detachment caused by physical adsorption.

[0065] According to some embodiments of the present invention, the carbon layer is formed by pyrolysis of a carbon source; the carbon source is obtained by reacting at least one of phenolic resin, vinylphosphonic acid, and hydroxymethylphosphonic acid with pyrophosphate, such that the carbon source contains pyrophosphate groups.

[0066] Among them, a large amount of pyrophosphate (P2O7) is exposed on the surface of NFPP crystals. 4- The crystal plane contains coordinatingly unsaturated Fe atoms. 2+ Active sites; pyrophosphate units in the carbon layer and P2O7 on the NFPP crystal plane 4- With identical bond lengths, bond angles, and spatial configurations, lattice orientation matching can be achieved; simultaneously, during pyrolysis, the electron-rich oxygen atoms at the ends of the P2O7 groups in the carbon layer interact with the Fe atoms on the NFPP surface. 2+A coordination reaction occurs, forming a stable PO-Fe covalent bond.

[0067] According to some embodiments of the present invention, the phase purity of the NFPP crystal nucleus is ≥99.5%, wherein Fe 2+ / Fe 3+ The molar ratio is (2.95~3.0):0.1, for example 2.95:0.1, 2.96:0.1, 2.97:0.1, 2.98:0.1, 2.99:0.1, 3:0.1. In the embodiments of the present invention, the carbon source can also provide directional pyrophosphate feeding for NFPP crystallization, thereby inhibiting pyrophosphate decomposition, giving NFPP high crystalline phase purity (≥99.5%), impurity phase content (≤0.2%), and almost complete suppression of impurity phases.

[0068] According to some embodiments of the present invention, the primary particle size of the NFPP crystal nucleus is 100~300nm, and the NFPP crystal nucleus is spherical.

[0069] According to some embodiments of the present invention, the carbon layer is a single layer with a thickness of 0.8~2.5 nm; the carbon content is 1.2%~2.8% of the mass of the NFPP crystal nucleus, for example, 1.2%, 1.4%, 1.6%, 1.8%, 2%, 2.2%, 2.4%, 2.6%, 2.8%; the phosphorus content in the carbon layer is 0.5at%~1.8at%, for example, 0.5at%, 0.6at%, 0.8at%, 1at%, 1.2at%, 1.4at%, 1.5at%, 1.6at%, 1.8at%.

[0070] According to some embodiments of the present invention, the carbon layer and the NFPP crystal nucleus are doubly anchored by lattice matching and PO-Fe covalent bonds, and the interfacial bonding strength is ≥12N / m.

[0071] According to some embodiments of the present invention, the carbon layer shedding rate of the sodium iron pyrophosphate cathode material after 1000 cycles at 1C is ≤0.3%. In the sodium iron pyrophosphate cathode material of the present invention, the carbon layer is anchored by lattice matching and PO-Fe covalent bonds, and the interfacial bonding strength is increased by more than 100% compared with physical coating, making the carbon layer less prone to shedding.

[0072] According to some embodiments of the present invention, the electronic conductivity of the sodium iron pyrophosphate cathode material is ≥1.2×10⁻⁶. - 5 S·cm -1 0.1C discharge capacity ≥107.8mAh·g -1 5C discharge capacity ≥ 91.5 mAh·g -1The capacity retention rate after 500 cycles at 1C is ≥93.9%. In the sodium iron pyrophosphate cathode material of this invention, the single-layer covalently anchored carbon layer can construct a continuous conductive network, which improves the electronic conductivity of the material by 1 to 2 orders of magnitude. At the same time, the carbon layer, which is doubly anchored by lattice matching and PO-Fe covalent bonds, is not easy to fall off, thereby greatly optimizing the rate performance and cycle capability.

[0073] The sodium iron pyrophosphate cathode material prepared by this invention has at least one of the following advantages: (1) Strong interface anchoring, carbon layer is not easy to fall off: Through the dual effect of lattice matching and PO-Fe covalent bond, the interface bonding strength is increased by more than 100% compared with physical coating. The carbon layer fall-off rate is ≤0.3% after 1000 cycles of 1C, which completely solves the problem of carbon layer fall-off and realizes the atomic-level strong anchoring of carbon layer and NFPP crystal nucleus. (2) High crystal phase purity and complete suppression of impurity phases: The carbon source provides directional pyrophosphate feeding for NFPP crystallization, suppressing pyrophosphate decomposition, with crystal phase purity ≥99.5% and impurity phase content ≤0.2%; (3) Significantly improved conductivity: A single-layer covalently anchored carbon layer constructs a continuous and stable conductive network, increasing the electronic conductivity by 1 to 2 orders of magnitude, reaching 1.2 × 10⁻⁶. -5 S·cm -1 In summary, the rate performance and fast charging capability have been significantly optimized. (4) The process is extremely simple and energy consumption is reduced: The preheating decomposition + final sintering one-step linkage process is adopted, which eliminates the need for step-by-step sintering and outer layer coating, and realizes the synergistic preparation of carbon layer preheating decomposition anchoring and NFPP synchronous crystallization. The process flow is shortened by 40% and energy consumption is reduced by more than 35%, which is suitable for industrial mass production. (5) Excellent long-cycle performance: The capacity retention rate after 500 cycles at 1C is ≥93.9%, which is far better than the existing technology of less than 80%, and the carbon layer does not fall off significantly, meeting the requirements of long-cycle use for large-scale energy storage.

[0074] Embodiments of the present invention also provide an application of the above-described sodium iron pyrophosphate cathode material or the sodium iron pyrophosphate cathode material prepared by the above-described preparation method in sodium-ion batteries.

[0075] The technical solution of the present invention will be further illustrated below with specific embodiments.

[0076] Example 1 The preparation method of the sodium iron pyrophosphate cathode material in this embodiment is as follows: S1, Preparation of phenolic resin-based carbon source (10g product scale): S11, Dissolution: Add 8.0g of thermoplastic phenolic resin (PF) and 40mL of mixed solvent to a three-necked flask, under N2 protection, and stir at 60℃ for 30min until completely dissolved; wherein, the solvent is anhydrous ethanol / dimethylformamide (DMF) = 7:3 (v / v). S12, Reaction: Slowly add a mixture of H4P2O7 and p-toluenesulfonic acid (PTSA) dropwise to the mixture obtained in S11 (completed in 10 min), heat to 85℃, and reflux for 4 h; continuously remove the generated water during the reaction (using a water separator) to promote the formation of POC bonds; wherein, H4P2O7 is an 85% H4P2O7 aqueous solution, freshly prepared or refrigerated, and the amount added is 8.6 g (molar ratio PF-OH: H4P2O7 = 1:0.55); the amount of p-toluenesulfonic acid added is 2.5 wt% and 0.2 g.

[0077] S13, post-treatment: Cool to room temperature, pour in 100 mL of deionized water, and a pale yellow solid precipitates; filter, wash 3 times with deionized water (to remove free pyrophosphate), and vacuum dry at 60℃ for 12 h to obtain phenolic resin-based carbon source.

[0078] S2: Weigh sodium acetate, ferrous oxalate, and ammonium dihydrogen phosphate according to the molar ratio of Na:Fe:P = 4:2.98:4.1; add the phenolic resin-based carbon source prepared in S1 (4.0 wt% of the total mass of sodium, iron, and phosphorus sources), PVP (0.5 wt% of the total mass of sodium, iron, and phosphorus sources), and deionized water (140% of the total mass of sodium, iron, and phosphorus sources), ball mill for 2.0 h to obtain a precursor slurry; dry at 95 °C for 10 h, pulverize through a 250 mesh sieve to obtain precursor powder; S3, the precursor powder was kept at 350℃ for 3 hours under a nitrogen atmosphere (80mL / min) to preheat and de-anchor the carbon source; S4. The powder after pre-pyrolysis of S3 is directly heated to 630℃ and held for 7 hours for sintering and crystallization. After the holding period, it is cooled to room temperature and passed through a 350-mesh sieve to obtain the cathode material.

[0079] The sodium iron pyrophosphate cathode material in this embodiment is spherical, with a primary particle size of 120-150 nm for NFPP crystal nuclei and a crystal phase purity of 99.6%. Fe 2+ / Fe 3+ The molar ratio is 2.95:0.1; the thickness of the carbon layer is 1.2~1.5nm, the carbon content is 1.4wt% of the mass of the NFPP crystal nucleus, the phosphorus content is 0.8at, and the interfacial bonding strength is ≥12.5N / m.

[0080] The crystal phase purity was measured using a pyrophosphate selective acid-base titration method: 0.1 g of the sample was accurately weighed, digested in a strongly alkaline, high-temperature, sealed environment until all phosphates dissolved; the pH of the system was adjusted to 3.8, and ZnSO4 was added to selectively precipitate P2O7. 4- (Only NFPP provides pyrophosphate; the heterophase has no P2O7) 4- ); standard NaOH titration releases hydrogen ions, calculate the total pyrophosphate mass of the sample; according to NFPP theory, P2O7 4- Theoretical ratio (Na4Fe3(PO4)2P2O7) molecules contain 1 group of P2O7 4- ), convert the theoretical mass of pure NFPP; crystal phase purity = measured NFPP mass / total sample mass × 100%.

[0081] Fe 2+ / Fe 3+ The molar ratio was measured by chemical titration (potassium dichromate oxidation titration): 0.1 g of the sample was accurately weighed, and digested in a closed system with dilute sulfuric acid at low temperature under inert nitrogen protection, completely isolating it from air to prevent Fe²⁺. + Oxidation; after removing the supernatant, titrate with standard K₂Cr₂O₇ solution using sodium diphenylamine sulfonate as an indicator to directly determine the Fe²⁺ content in the system. + Amount of substance n (Fe²) + Another equal amount of sample was digested in an open-air concentrated acid at high temperature, and all the iron was converted into Fe³⁺. + After reduction, titration yields the total amount of iron, n(total Fe); Calculate: nFe 3+ =n(Fe 总 )-n(Fe 2+ ), ultimately yielding Fe² + / Fe³ + Molar ratio.

[0082] Phosphorus content was determined using X-ray photoelectron spectroscopy (XPS): The sample powder was pressed into a pellet, and the full XPS spectrum and fine spectra of P, C, Fe, and O on the sample surface were acquired using an Al-Kα monochromatic source. Peak fitting of each element's characteristic peaks was performed using software, and the relative atomic concentrations of P, C, and O were read. The phosphorus content in the carbon layer was calculated as: (Number of phosphorus atoms / Total number of C+P+O atoms) × 100%, in at%. The measurement depth was 2–5 nm, characterizing only the elemental composition of the surface carbon coating layer and unaffected by phosphorus signals from the internal NFPP crystals.

[0083] Example 2 The only difference between this embodiment and Example 1 is that the amount of phenolic resin-based carbon source added is 6.0 wt%, and the rest is the same as in Example 1.

[0084] The sodium iron pyrophosphate cathode material in this embodiment is spherical, with a primary particle size of 120-150 nm for NFPP crystal nuclei and a crystal phase purity of 99.7%. Fe 2+ / Fe 3+ The molar ratio is 2.97:0.1; the thickness of the carbon layer is 1.5~1.8nm, the carbon content is 1.8wt% of the mass of the NFPP crystal nucleus, the phosphorus content is 1.1at%, and the interfacial bonding strength is ≥13.2N / m.

[0085] Example 3 The preparation method of the sodium iron pyrophosphate cathode material in this embodiment is as follows: S1, Preparation of vinylphosphonic acid-based carbon source (10g product scale): S11, Dissolution: Under an anhydrous, nitrogen-protected atmosphere, add 6.5 g of vinylphosphonic acid (VPA, 98%) and 50 mL of anhydrous tetrahydrofuran to a dried three-necked flask, and stir in an ice bath at 0 °C for 15 min. S12, condensation reaction: Dicyclohexylcarbodiimide (DCC, 1.84 g, 15 mol% of vinylphosphonic acid) was slowly added to the solution obtained in S11; then H4P2O7 (85%, 9.87 g, molar ratio VPA:H4P2O7 = 1:0.8) was added dropwise over 15 min; the reaction was stirred at 25 °C for 6 h, resulting in a white DCC-urea precipitate. S13, purification: Filter to remove precipitate, concentrate the filtrate by rotary evaporation (40℃, -0.08MPa) to obtain a pale yellow viscous liquid; recrystallize twice with ethyl acetate / n-hexane, and dry under vacuum at 40℃ for 8h to obtain a vinylphosphonic acid carbon source.

[0086] S2: Weigh sodium carbonate, ferrous sulfate, and sodium pyrophosphate according to the molar ratio of Na:Fe:P = 4:3.0:4.0; add the vinylphosphonic acid-based carbon source prepared in S1 (3.5% of the total mass of sodium, iron, and phosphorus sources), PEG-6000 (0.7% of the total mass of sodium, iron, and phosphorus sources), and deionized water (140% of the total mass of sodium, iron, and phosphorus sources), ball mill for 2.5 h to obtain a precursor slurry; dry at 105℃ for 9 h, pulverize and pass through a 200-mesh sieve to obtain precursor powder; S3, the precursor powder was kept at 360℃ for 3 hours in an argon atmosphere (100mL / min) to preheat and de-anchor the carbon source; S4. The powder after pre-pyrolysis of S3 is directly heated to 600℃ and held for 8 hours for sintering and crystallization. After the holding period, it is cooled to room temperature and passed through a 300-mesh sieve to obtain the cathode material.

[0087] like Figure 1As shown, the sodium iron pyrophosphate cathode material in this embodiment is spherical, with a primary particle size of 130-170 nm for the NFPP crystal nuclei; its crystal phase purity is 99.5%, and Fe... 2+ / Fe 3+ The molar ratio is 2.96:0.1; For example... Figure 2 As shown, the thickness of the carbon layer is 1.0~1.3nm, the carbon content is 1.3wt% of the mass of the NFPP crystal nucleus, the phosphorus content is 0.7at%, and the interfacial bonding strength is ≥12.1N / m.

[0088] Example 4 The only difference between this embodiment and Example 3 is that the amount of vinylphosphonic acid-based carbon source added is 5.5 wt%, and the rest is the same as in Example 3.

[0089] The sodium iron pyrophosphate cathode material in this embodiment is spherical, with a primary particle size of 130-170 nm for NFPP crystal nuclei and a crystal phase purity of 99.6%. (Fe...) 2+ / Fe 3+ The molar ratio is 2.98:0.1; the thickness of the carbon layer is 1.3~1.6nm, the carbon content is 1.6wt% of the mass of the NFPP crystal nucleus, the phosphorus content is 1.0at%, and the interfacial bonding strength is ≥12.8N / m.

[0090] Example 5 The preparation method of the sodium iron pyrophosphate cathode material in this embodiment is as follows: S1, Preparation of hydroxymethylphosphonic acid-based carbon source (10g product scale): S11, Mixing: Add 5.8 g of hydroxymethylphosphonic acid (HMPA, 98%) and 30 mL of mixed solvent (deionized water / ethylene glycol = 1:1 (v / v)) to a three-necked flask, and stir at 70 °C for 20 min until clear under N2 protection; S12, Reaction: Pyrophosphate (H4P2O7, 85%, 11.7g, molar ratio HMPA:H4P2O7=1:1.1) and concentrated sulfuric acid (0.087mL, 1wt% of the total mass of pure hydroxymethylphosphonic acid and pure pyrophosphate) were added dropwise to the solution obtained in S11. The temperature was raised to 120℃, and the dehydration reaction was carried out for 3h (the water in the system was evaporated). The temperature was then raised to 150℃ and kept at that temperature for 1h to strengthen the formation of POP bonds. S13, collection: cooled to 80℃, 80mL of isopropanol was poured in, and a white powder precipitated; filtered, washed twice with isopropanol, and dried under vacuum at 80℃ for 10h to obtain hydroxymethylphosphonic acid carbon source.

[0091] S2: Sodium bicarbonate, ferrous chloride, and ammonium dihydrogen phosphate were weighed according to the molar ratio of Na:Fe:P = 4:3.02:4.2. The hydroxymethylphosphonic acid-based carbon source prepared in S1 (4.5% of the total mass of sodium, iron, and phosphorus sources), PVP (0.4% of the total mass of sodium, iron, and phosphorus sources), and deionized water (140% of the total mass of sodium, iron, and phosphorus sources) were added to the mixture. The mixture was ball-milled for 1.8 hours to obtain a precursor slurry. The slurry was dried at 85°C for 11 hours and then pulverized through a 300-mesh sieve to obtain the precursor powder. S3, the precursor powder was kept at 380℃ for 2.5h in a nitrogen-argon mixed atmosphere (60mL / min) to preheat and de-anchor the carbon source; S4. The powder after pre-pyrolysis of S3 is directly heated to 660℃ and held for 6 hours for sintering and crystallization. After the holding period, it is cooled to room temperature and passed through a 400-mesh sieve to obtain the cathode material.

[0092] The sodium iron pyrophosphate cathode material in this embodiment is spherical, with a primary particle size of 110-140 nm for NFPP crystal nuclei and a crystal phase purity of 99.7%. Fe 2+ / Fe 3+ The molar ratio is 2.99:0.1; the thickness of the carbon layer is 1.4~1.7nm, the carbon content is 1.7wt% of the mass of the NFPP crystal nucleus, the phosphorus content is 1.2at%, and the interfacial bonding strength is ≥13.5N / m.

[0093] Example 6 The only difference between this embodiment and Example 5 is that the amount of hydroxymethylphosphonic acid carbon source added is 6.5 wt%, and the rest is the same as Example 5.

[0094] The sodium iron pyrophosphate cathode material in this embodiment is spherical, with a primary particle size of 110-140 nm for NFPP crystal nuclei and a crystal phase purity of 99.8%. Fe 2+ / Fe 3+ The molar ratio is 3.00:0.1; the thickness of the carbon layer is 1.7~2.0nm, the carbon content is 2.0wt% of the mass of the NFPP crystal nucleus, the phosphorus content is 1.5at, and the interfacial bonding strength is ≥14.0N / m.

[0095] Comparative Example 1 The preparation method of the sodium iron pyrophosphate cathode material in this comparative example is as follows: S1, weigh out sodium hydroxide, ferrous oxalate and aminotrimethylenephosphonic acid according to Na:Fe:P=4:3:4, dissolve them, evaporate them to dryness and dry them at 80℃ for 10h; S2 was pre-sintered at 350°C for 8 hours under a nitrogen atmosphere, and then sintered again at 600°C for 8 hours; the NFPP / carbon material was obtained by cooling and grinding.

[0096] Comparative Example 2 In this comparative example, glucose was used to replace the carbon source in Example 1, and the amount of carbon source added was 6.0 wt%. The rest of the process was completely consistent with Example 1, and a physically coated NFPP / carbon material was obtained.

[0097] The cathode materials prepared in Examples 1-6 and Comparative Examples 1-2 were assembled into coin cells. Specifically, the cathode material, acetylene black, and PVDF were weighed and mixed at a mass ratio of 8:1:1, and N was added. The methylpyrrolidone solution (NMP) was homogenized, and then the slurry was uniformly coated onto aluminum foil. It was dried in a vacuum drying oven at 80°C for 6 hours. The positive electrode was obtained by rolling and cutting (14 mm in diameter). A sodium metal sheet was used as the counter electrode. 1 mol / L NaClO4 was used as the electrolyte. The electrolyte was 1 M sodium perchlorate (NaClO4) dissolved in a 1:1 volume ratio ethylene carbonate / propylene carbonate (EC / PC) system, with 5 wt% fluoroethylene carbonate (FEC) added. The separator was made of glass fiber. The cells were assembled into a 2032 button cell in an argon-filled glove box.

[0098] The performance of the cathode materials and sodium-ion batteries prepared in Examples 1-6 and Comparative Examples 1-2 was tested: Interfacial bonding strength test: The interfacial bonding strength between the carbon layer and the crystal was tested using a nano-scratch tester. A diamond triangular pyramidal probe was used, with a linearly increasing load of 0-50 mN, a probe loading rate of 10 mN / min, and a scratch length of 500 μm. The critical load Lc at which the carbon layer peeled off was recorded, and the interfacial bonding strength was calculated using the following formula: Interfacial bonding strength (N / m) = Lc / d, where Lc is the critical peel load and d is the carbon layer thickness.

[0099] Carbon layer shedding rate calculation: Before cycling: test the total carbon content C1 of the material (organic elemental analyzer / TGA). After 1000 cycles at 1C: disassemble the battery, remove the electrode, wash and dry it, and measure the residual carbon content C2. Calculate using the following formula: Carbon layer shedding rate (%) = (C1) / (C2) C2) / C1×100%.

[0100] The following battery performance tests were conducted at room temperature (25°C): 0.1C discharge specific capacity test: The first discharge specific capacity was measured by constant current charging and discharging at a current density of 0.1C (1C=120mAh / g) within a voltage range of 1.5~3.8V.

[0101] Rate performance test: Charge and discharge at current densities of 0.1C and 5C respectively, record the discharge specific capacity at different rates, and calculate the rate retention rate.

[0102] Cyclic performance test: Cycle 500 times with a current density of 1C in the voltage range of 1.5~3.8V, record the discharge specific capacity at different number of cycles, and calculate the capacity retention rate after 500 cycles.

[0103] The performance test results of the cathode materials and sodium-ion batteries of each embodiment and comparative example are shown in Table 1.

[0104] Table 1 Performance test results of the cathode materials and sodium-ion batteries in each embodiment and comparative example.

[0105] As can be seen from Example 1 and Comparative Example 1, Comparative Example 1 uses a one-step physical mixing and sintering of common organophosphonic acids, without constructing a lattice-matched and PO-Fe covalent anchoring structure. It only performs physical adsorption coating and fails to achieve a strong interfacial bond between the carbon layer and the crystal nucleus, resulting in poor electrical performance.

[0106] As can be seen from Example 1 and Comparative Example 2, Comparative Example 2 uses a common glucose carbon source, which fails to form lattice matching and covalent anchoring with the NFPP crystal plane. It is only a conventional physical coating, resulting in poor electrical performance.

[0107] Compared to Example 2, the reduced amount of phenolic resin-based carbon source in Example 1 leads to relatively insufficient carbon layer continuity and a relatively decreased lattice matching, resulting in a relative decrease in interfacial bonding strength and electronic conductivity, and consequently, a relative decline in electrical properties. However, the carbon source amounts in Examples 1 and 2 are both within the aforementioned design range, and the interfacial bonding strength, electronic conductivity, and electrical properties are all significantly improved compared to the comparative example.

[0108] Compared to Example 1, Example 3 uses a vinylphosphonic acid-based carbon source, which has a relatively low pyrophosphate group content. This results in a relatively reduced amount of PO-Fe covalent bonds and insufficient pyrophosphate feeding, leading to a relative decrease in crystal phase purity, cycle stability, and electrical performance. However, the crystal phase purity, cycle stability, and electrical performance of Examples 1 and 3 are significantly improved compared to the comparative example.

[0109] As can be seen from Examples 3 and 4, reducing the amount of vinylphosphonic acid-based carbon source leads to incomplete carbon layer coating and reduced lattice matching coverage, resulting in interrupted electron transport paths, relatively shortened cycle life, and relatively decreased electrical performance. However, the amount of carbon source added in Examples 3 and 4 is within the above-mentioned design range, and the material performance and battery performance are significantly improved compared to the comparative example.

[0110] As can be seen from Examples 5 and 6, reducing the amount of hydroxymethylphosphonic acid-based carbon source leads to insufficient preheating and anchoring, insufficient covalent bond density, resulting in easy carbon layer detachment, weakened impurity phase suppression effect, and decreased electrical performance. However, the amount of carbon source added in Examples 3 and 4 is within the above design range, and the material performance and battery performance are significantly improved compared to the comparative examples.

[0111] The above description is only a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any changes or substitutions that can be easily conceived by those skilled in the art within the scope of the technology disclosed in the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for preparing a carbon-layer covalently anchored sodium iron pyrophosphate cathode material, characterized in that, include: S1, Prepare a carbon source, wherein the carbon source contains pyrophosphate groups; S2, weigh out sodium source, iron source and phosphorus source, add carbon source, dispersant and solvent obtained in S1, and ball mill to obtain precursor slurry; dry the precursor slurry to obtain precursor powder; S3, the precursor powder in S2 is pre-pyrolyzed with a carbon source to obtain a carbon layer pre-anchored precursor. S4, the carbon layer pre-anchored precursor described in S3 is directly heated and sintered to crystallize the NFPP crystal nuclei and stabilize the carbon layer. After calcination, the material is cooled and pulverized to obtain a carbon-covalently anchored sodium iron pyrophosphate cathode material.

2. The preparation method according to claim 1, characterized in that, The carbon source includes at least one of the following: phenolic resin-based carbon source, vinyl phosphonic acid-based carbon source, and hydroxymethyl phosphonic acid-based carbon source.

3. The preparation method according to claim 1, characterized in that, In step S2, the amount of carbon source added is 3.5% to 7.5% of the total mass of the sodium source, iron source, and phosphorus source.

4. The preparation method according to claim 1, characterized in that, In step S3, the pyrolysis temperature of the carbon source preheating is 300~400℃.

5. The preparation method according to claim 1, characterized in that, In step S4, the sintering temperature is 580~680℃.

6. A carbon-layer covalently anchored sodium iron pyrophosphate cathode material, characterized in that, The preparation method according to any one of claims 1 to 5 is used to prepare the NFPP crystal nucleus and a carbon layer coated on the NFPP crystal nucleus.

7. The sodium iron pyrophosphate cathode material according to claim 6, characterized in that, The NFPP crystal nucleus has a phase purity of ≥99.5%, wherein Fe 2+ / Fe 3+ The molar ratio is (2.95~3.0):0.1; and / or, the primary particle size of the NFPP crystal nucleus is 100~300nm, and the NFPP crystal nucleus is spherical.

8. The sodium iron pyrophosphate cathode material according to claim 6, characterized in that, The thickness of the carbon layer is 0.8~2.5nm, the carbon content is 1.2%~2.8% of the mass of the NFPP crystal nucleus, and the phosphorus content in the carbon layer is 0.5at%~1.8at.

9. The sodium iron pyrophosphate cathode material according to claim 6, characterized in that, The electronic conductivity of the sodium iron pyrophosphate cathode material is ≥1.2×10⁻⁶. -5 S·cm -1 The capacity retention rate is ≥93.9% after 500 cycles at 1C, and the carbon layer shedding rate is ≤0.3% after 1000 cycles at 1C.

10. The application of a sodium iron pyrophosphate cathode material prepared by any one of claims 1 to 5 or the sodium iron pyrophosphate cathode material according to any one of claims 6 to 9 in a sodium-ion battery.

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