Positive electrode material and preparation method and application thereof

Abstract

The invention belongs to the technical field of positive electrode materials, and particularly relates to a positive electrode material and a preparation method and application thereof. The positive electrode material comprises a substrate and a coating layer. The chemical formula of the matrix is Na4Fe3-3x (PO4) 2P2O7, and the value range of x is 0.01 to 0.06; the coating layer comprises an amorphous carbon layer and a graphene layer which are arranged from the substrate side to the outside in sequence. Through collaborative design of a graphene coating technology and iron defects (Fe vacancies), the three bottlenecks of low intrinsic electron conductivity, insufficient active sites and slow ion diffusion kinetics of the material can be solved at the same time, and the material performance is improved in a breakthrough manner, so that the positive electrode material has high working voltage and structural stability, and the performance of the positive electrode material is improved. The room-temperature rate performance is remarkably improved, and the cycling stability and the reversible capacity are synchronously improved. And meanwhile, the electrochemical performance of the button sodium ion battery can be improved by a ball-milling and sanding combined solid-phase synthesis method.

Description

Technical Field

[0001] This invention belongs to the field of new energy materials technology, and specifically relates to a cathode material, its preparation method, and its application. Background Technology

[0002] Large-scale energy storage systems are crucial for the efficient utilization of renewable energy. Sodium-ion batteries, with their similar intercalation chemistry to lithium-ion batteries and advantages of abundant resources and low cost, are considered an ideal alternative to lithium-ion systems. However, the development of sodium-ion batteries still faces significant challenges, including low energy and power density, slow sodium-ion kinetics, and insufficient cycle stability.

[0003] Given the crucial role of cathode materials, researchers have developed various cathode systems with diverse crystal structures, including transition metal oxides, polyanionic compounds, Prussian blue analogs, and organic materials, to advance cathode material technology. Among these, the sodium superionic conductor type (NASICON) polyanionic compound iron-based mixed phosphate cathode material Na4Fe3(PO4)2(P2O7) (NFPP) stands out due to its structural stability (volume change <4%) and high efficiency (129 mAh g / L). -1 It has attracted much attention due to its theoretical capacity and operating potential of 3.2V. NFPP has an open framework structure, connected by three FeO6 octahedra and two PO4 tetrahedra sharing vertices, and further bridged by [P2O7] groups along the a-axis. Adjacent [Fe3P2O] groups... 13 The ∞ layers are interconnected along the a-axis by P2O7 groups, a structural feature of which is Na + Transport along the b-axis provides a wide ion channel, enabling three-dimensional ion transport and a low sodium ion diffusion barrier.

[0004] Despite some progress in NFPP research, significant breakthroughs in its performance have yet to be achieved, primarily due to two major technical bottlenecks: First, the formation of NFPP is highly sensitive to synthesis temperature and raw materials; improper process control can easily lead to the formation of NaFePO4 and Na2FeP2O7 impurity phases. Second, the extremely low electronic / ionic conductivity of NFPP severely restricts its capacity and power density. Furthermore, the relatively large radius and mass of sodium ions result in slow electrochemical kinetics, and the cathode material is prone to severe volume changes during charge and discharge, thus affecting its rate performance and cycle stability. Finally, the environmental temperature adaptability and redox potential characteristics of some phosphate-based polyanionic compounds make it difficult for ordinary electrolytes to fully utilize their sodium storage capacity under extreme conditions (such as high and low temperature environments or high operating voltages). While the design of carbon-modified NFPP composites holds promise for improving electrochemical performance, challenges remain in capacity, rate performance, and cycle stability; current performance still cannot meet practical application requirements.

[0005] Therefore, there is an urgent need to provide a cathode material with good redox activity and electronic conductivity, which can enable sodium-ion batteries to have good rate performance and cycle performance. Summary of the Invention

[0006] The present invention aims to solve one or more technical problems existing in the prior art, and at least provide a beneficial alternative. Specifically, the present invention provides a cathode material with good redox activity and electronic conductivity, which enables sodium-ion batteries to have good rate performance and cycle performance.

[0007] The inventive concept of this invention: The positive electrode material of this invention includes a matrix and a coating layer, wherein the chemical formula of the matrix is ​​Na₄Fe₂O₃. 3-3x (PO4)2P2O7, where x ranges from 0.01 to 0.06; the coating layer comprises a carbon layer and a graphene layer arranged sequentially from the substrate side outwards.

[0008] This invention innovatively introduces graphene coating technology and synergistic design with iron defects (Fe vacancies), simultaneously addressing three major bottlenecks: low intrinsic electronic conductivity, insufficient active sites, and slow ion diffusion kinetics, resulting in a breakthrough improvement in material performance. Graphene coating on the surface of active particles constructs a three-dimensional continuous conductive network structure. This conductive network possesses a high specific surface area and excellent electronic conductivity, providing an efficient electron transport channel for the electrode material. Furthermore, the close indirect contact between graphene and the active material significantly enhances charge transport kinetics and redox activity. Simultaneously, iron defects induce local lattice distortion, reducing Na+ ionization. + A diffusion barrier accelerates ion transport. The amorphous carbon layer suppresses excessive grain growth during high-temperature calcination, providing structural stability. The synergistic effect of graphene coating technology and iron defects, combined with the amorphous carbon layer, enables this cathode material to maintain high operating voltage and structural stability while significantly improving room-temperature rate performance, and simultaneously enhancing cycle stability and reversible capacity.

[0009] Therefore, a first aspect of the present invention provides a cathode material.

[0010] Specifically, the positive electrode material includes a matrix and a coating layer; The chemical formula of the matrix is ​​Na4Fe 3-3x (PO4)2P2O7, the value of x ranges from 0.01 to 0.06; The coating layer comprises an amorphous carbon layer and a graphene layer arranged sequentially from the substrate side outwards.

[0011] Preferably, the value of x is in the range of 0.01-0.05.

[0012] Preferably, the carbon content in the cathode material is 4.5-11 wt%; more preferably, the carbon content in the cathode material is 5-10 wt%.

[0013] Preferably, the positive electrode material has a porous structure with a pore size of 9-110 nm; more preferably, the positive electrode material has a porous structure with a pore size of 10-100 nm.

[0014] Preferably, the positive electrode material has an irregular blocky structure.

[0015] Preferably, the particle size of the positive electrode material is 0.7-16 μm; more preferably, the particle size of the positive electrode material is 0.8-15 μm.

[0016] Preferably, the graphene layer is a reduced graphene oxide layer.

[0017] A second aspect of the present invention provides a method for preparing the cathode material described in the first aspect of the present invention.

[0018] Specifically, the preparation method of the cathode material includes the following steps: (1) Iron source, sodium source, phosphorus source, carbon source and solvent are mixed to obtain a dispersion; the dispersion is ball-milled to obtain a first precursor; (2) The first precursor and graphene are mixed and milled to obtain the second precursor; then calcined to obtain the cathode material.

[0019] Specifically, in the initial ball milling stage, the active material precursor and an organic carbon source (such as sucrose or glucose) are ball milled together. The mechanical energy generated by the ball milling causes the organic carbon source molecules to uniformly coat the surface of the active material particles and their internal pores, simultaneously forming a close physical contact interface. Subsequently, graphene raw material is introduced during sand milling. The high shear force of sand milling effectively disperses the graphene, allowing it to collide with the pre-coated carbon source active material particles in the liquid medium. Due to graphene's huge specific surface area and abundant surface functional groups, it spontaneously coats the particle surface through electrostatic adsorption, van der Waals forces, or hydrogen bonding interactions, ultimately resulting in a sequentially coated structure.

[0020] Preferably, in step (1), the iron source includes at least one of ferrous sulfate heptahydrate, ferric citrate, ferric oxalate, ferric nitrate nonahydrate, ferric sulfate, and ferric phosphate.

[0021] Preferably, in step (1), the sodium source is at least one of sodium carbonate, sodium pyrophosphate, sodium nitrate, sodium sulfate, disodium bicarbonate, sodium bicarbonate, and sodium dihydrogen phosphate.

[0022] Preferably, in step (1), the phosphorus source is at least one of sodium dihydrogen phosphate, diammonium hydrogen phosphate, ammonium dihydrogen phosphate, and phosphoric acid.

[0023] Preferably, in step (1), the carbon source is at least one of glucose, citric acid, sucrose, polyethylene glycol, and urea.

[0024] Preferably, in step (1), the solvent includes water.

[0025] Preferably, the amount of solvent used accounts for 35-55% of the total solid content in the system; more preferably, the amount of solvent used accounts for 40-50% of the total solid content in the system.

[0026] Preferably, in step (1), the molar ratio of sodium source, iron source, and phosphorus source is Na:Fe:P = 4:(3-3x):4, ​​and the value of x is in the range of 0.01-0.06; more preferably, the molar ratio of sodium source, iron source, and phosphorus source is Na:Fe:P = 4:(3-3x):4, ​​and the value of x is in the range of 0.01-0.05; even more preferably, the molar ratio of sodium source, iron source, and phosphorus source is Na:Fe:P = 4:(3-3x):4, ​​and the value of x is in the range of 0.03.

[0027] Preferably, in step (1), the amount of carbon source added is 4.5-11% of the theoretical product mass of the cathode material; more preferably, the amount of carbon source added is 5-10% of the theoretical product mass of the cathode material.

[0028] Preferably, in step (1), the ball milling speed is 320-550 rpm and the ball milling time is 3.5-6.5 h; more preferably, the ball milling speed is 350-500 rpm and the ball milling time is 4-6 h.

[0029] Preferably, in step (2), the graphene includes graphene oxide.

[0030] Preferably, the graphene oxide is a graphene oxide solution.

[0031] Preferably, the graphene oxide solution is an aqueous solution of graphene oxide.

[0032] Preferably, the concentration of the graphene oxide solution is 4-6 mg / mL; more preferably, the concentration of the graphene oxide solution is 4.5-5.5 mg / mL; and even more preferably, the concentration of the graphene oxide solution is 5 mg / mL.

[0033] Preferably, in step (2), the amount of graphene added is 4.5-16.5% of the theoretical product mass of the cathode material; more preferably, the amount of graphene added is 5-15% of the theoretical product mass of the cathode material.

[0034] Preferably, in step (2), the rotation speed of the sand mill is 900-2200 rpm and the sand milling time is 1.8-5.5 h; more preferably, in step (2), the rotation speed of the sand mill is 1000-2000 rpm and the sand milling time is 2-5 h.

[0035] Preferably, in step (2), the second precursor is first dried, ground, and then calcined.

[0036] Preferably, the drying temperature is 80-120℃ and the drying time is 13-17h; more preferably, the drying temperature is 90-110℃ and the drying time is 14-16h; even more preferably, the drying temperature is 100℃ and the drying time is 15h.

[0037] Preferably, an oven is used for drying.

[0038] Preferably, in step (2), the calcination temperature is 350-650℃ and the calcination time is 6.5-12.5h; more preferably, the calcination temperature is 400-600℃ and the calcination time is 7-12h.

[0039] Preferably, the heating rate of the calcination is 2.7-5.5℃ / min; more preferably, the heating rate of the calcination is 3-5℃ / min.

[0040] Preferably, the calcination equipment is a tubular furnace.

[0041] Preferably, the protective atmosphere for calcination is any one of Ar, N2, H2 / Ar, and H2 / N2.

[0042] A third aspect of the present invention provides a sodium-ion battery. Specifically, the sodium-ion battery includes the positive electrode material described in the first aspect of the present invention.

[0043] Compared with the prior art, the beneficial effects of the technical solution provided by the present invention are as follows: (1) This invention innovatively introduces the synergistic design of graphene coating technology and iron defects (Fe vacancies). Graphene is coated on the surface of active particles to construct a three-dimensional continuous conductive network structure. This conductive network has a high specific surface area and excellent electronic conductivity, providing an efficient electron transport channel for the electrode material. At the same time, graphene forms a tight contact interface with the active material, significantly improving charge transport kinetics and redox activity. In addition, iron defects induce local lattice distortion, reducing Na+ ionization. + A diffusion barrier accelerates ion transport. The amorphous carbon layer, during high-temperature calcination, inhibits excessive grain growth and provides structural stability. The synergistic effect of graphene coating technology and iron defects, combined with the amorphous carbon layer, simultaneously addresses three major bottlenecks: low intrinsic electronic conductivity, insufficient active sites, and slow ion diffusion kinetics. This results in a breakthrough improvement in material performance, enabling the cathode material to maintain high operating voltage and structural stability while significantly improving room-temperature rate performance, and simultaneously enhancing cycle stability and reversible capacity.

[0044] (2) The battery prepared by the cathode material of the present invention has high coulombic efficiency and good rate cycle performance, indicating that it has good structural stability. The structure of the material is not damaged or collapsed, the channels for sodium ion insertion and extraction are maintained for a long time, and the active material will not "lose contact" with the electrolyte. Therefore, the capacity decay is very slow.

[0045] (3) The cathode material of the present invention adopts the ball milling and sand milling solid phase synthesis method to regulate the microstructure of the cathode material, which can change the internal electron density distribution of the material and significantly improve the crystal structure stability of the iron-based mixed phosphate cathode material, so that it can maintain excellent structural integrity during long-term cycling, thereby obtaining stable electrochemical performance.

[0046] (4) The raw materials involved in this invention are cheap and readily available, the process is simple and the preparation cycle is short, which meets the requirements of energy conservation, emission reduction and green chemistry. Moreover, the daily output of the laboratory-grade products is high, which is conducive to industrial-scale application. Attached Figure Description

[0047] Figure 1 This is a scanning electron microscope image of the positive electrode material in Embodiment 1 of the present invention; Figure 2 The rate performance diagram shows the coin cell sodium-ion battery prepared with the cathode material of Example 1 and Comparative Example 1 of this invention. Figure 3 The image shows the cycle stability of the coin cell sodium-ion battery prepared using the cathode material of Example 1 and Comparative Example 1 of this invention. Detailed Implementation

[0048] To enable those skilled in the art to more clearly understand the technical solutions described in this invention, the following embodiments are provided for illustration. It should be noted that the following embodiments do not constitute a limitation on the scope of protection claimed by this invention.

[0049] Unless otherwise specified, the raw materials, reagents or devices used in the following examples are available from conventional commercial sources or can be obtained by existing known methods.

[0050] In Examples 1-3, the stirring speed was 500-800 rpm. Within this range, there was no difference in the results, and it only served to improve the mass transfer rate.

[0051] Example 1 This embodiment provides a positive electrode material, the preparation method of which is as follows: S1. Preparation of raw material dispersion: Add 100mL of pure water to a beaker, then add 12.538g of ammonium dihydrogen phosphate and stir to dissolve. Add 43.887g of ferric phosphate and 21.199g of sodium carbonate and stir vigorously at 500-800rpm. After the dispersion is uniform, add 3.049g of glucose and 3.049g of sucrose as carbon sources in sequence and continue stirring for 15min to obtain a uniformly dispersed solution. S2. Coarse grinding and refining: The dispersion obtained in S1 is placed in a grinding jar and loaded into a planetary ball mill. It is ball-milled at 500 rpm for 4 hours to obtain the first precursor. S3. Preparation of precursor materials: Graphene oxide was placed in deionized water and ultrasonically mixed to form a graphene oxide solution with a concentration of 5 mg / mL. 124 mL of graphene oxide solution was added to the first precursor, and then the mixture was transferred to a sand mill and milled at 1200 r / min for 3 h to obtain the second precursor. The second precursor was placed in a 100℃ oven and baked for 15 h, and the dried powder was then ground. S4. Preparation of cathode material: The powder obtained after grinding in S3 is placed in a ceramic boat and calcined at 550℃ under N2 protection conditions with a heating rate of 5℃ / min for 10 hours to obtain a graphene-modified iron-based mixed phosphate cathode material (denoted as Na4Fe). 2.91 (PO4)2P2O7 / C / rGO).

[0052] Example 2 This embodiment provides a positive electrode material, the preparation method of which is as follows: S1. Preparation of raw material dispersion: Add 100mL of pure water to a beaker, then add 12.538g of ammonium dihydrogen phosphate and stir to dissolve. Add 43.887g of ferric phosphate and 21.199g of sodium carbonate to the mixture and stir vigorously at 500-800rpm. After the mixture is evenly dispersed, add 6.21g of glucose as a carbon source and continue stirring for 15min to obtain a uniformly dispersed solution. S2. Coarse grinding and refining: The dispersion obtained in S1 is placed in a grinding jar and loaded into a planetary ball mill. It is ball-milled at 450 rpm for 4 hours to obtain the first precursor. S3. Preparation of precursor materials: Graphene oxide was placed in deionized water and ultrasonically mixed to form a graphene oxide solution with a concentration of 5 mg / mL. 62 mL of graphene oxide solution was added to the first precursor, and then the mixture was transferred to a sand mill and milled at 1500 r / min for 2.5 h to obtain the second precursor. The second precursor was placed in a 100℃ oven and baked for 15 h, and the dried powder was then ground. S4. Preparation of cathode material: The powder obtained after grinding in S3 was placed in a ceramic boat and calcined at 600℃ at a rate of 3℃ / min under a protective atmosphere of H2 / Ar (volume ratio of H2 to Ar is 5:95) for 10 hours to obtain a graphene-modified iron-based mixed phosphate cathode material (denoted as Na4Fe). 2.91 (PO4)2P2O7 / C / rGO).

[0053] Example 3 This embodiment provides a positive electrode material, the preparation method of which is as follows: S1. Preparation of raw material dispersion: Add 100mL of pure water to a beaker, then add 12.538g of ammonium dihydrogen phosphate and stir to dissolve. Add 43.887g of ferric phosphate and 21.199g of sodium carbonate and stir vigorously at 500-800rpm. After the dispersion is uniform, add 3.049g of carbon source citric acid and 3.049g of sucrose and continue stirring for 15min to obtain a uniformly dispersed solution. S2. Coarse grinding and refining: The dispersion obtained in S1 is placed in a grinding jar and loaded into a planetary ball mill. It is ball-milled at 400 rpm for 5 hours to obtain the first precursor. S3. Preparation of precursor materials: Graphene oxide was placed in deionized water and ultrasonically mixed to form a graphene oxide solution with a concentration of 5 mg / mL. 186 mL of graphene oxide solution was added to the first precursor, and then the mixture was transferred to a sand mill and milled at 1200 r / min for 3 h to obtain the second precursor. The second precursor was placed in a 100℃ oven and baked for 15 h, and the dried powder was then ground. S4. Preparation of cathode material: The powder obtained after grinding in S3 was placed in a ceramic boat and calcined at 550℃ for 10 hours under a protective atmosphere of H2 / Ar (H2 to Ar volume ratio of 5:95) at a rate of 4℃ / min. This yielded a graphene-modified iron-based mixed phosphate cathode material (denoted as Na4Fe). 2.91 (PO4)2P2O7 / C / rGO).

[0054] Comparative Example 1 Comparative Example 1 provides a method for preparing a cathode material, which differs from Example 1 in that the molar ratio of Na:Fe:P in the Na source, Fe source and phosphorus source is 4:3:4, and no graphene is added.

[0055] The preparation method of the cathode material in Comparative Example 1 is as follows: S1. Preparation of raw material dispersion: Add 100mL of pure water to a beaker, then add 11.503g of ammonium dihydrogen phosphate and stir to dissolve. Add 45.245g of ferric phosphate and 21.199g of sodium carbonate and stir vigorously. After the dispersion is uniform, add 3.049g of glucose and 3.049g of sucrose as carbon sources and continue stirring for 15min to obtain a uniformly dispersed solution. S2. Coarse grinding and refining: The dispersion obtained in S1 is placed in a grinding jar and loaded into a planetary ball mill. It is ball-milled at 500 rpm for 4 hours to obtain the first precursor. S3. Preparation of precursor materials: The first precursor is transferred to a sand mill and milled at 1200 rpm for 3 hours to obtain the second precursor; the second precursor is placed in a 100℃ oven and baked for 15 hours, and the dried powder is ground. S4. Preparation of cathode material: The powder obtained after grinding in S3 is placed in a ceramic boat and calcined at 550°C at a rate of 5°C / min under a N2 protective atmosphere for 10 hours to obtain the iron-based mixed phosphate cathode material (denoted as Na4Fe3(PO4)2P2O7 / C).

[0056] Comparative Example 2 Comparative Example 2 provides a method for preparing a cathode material, which differs from Example 1 in that graphene is not added, but otherwise it is the same as Example 1.

[0057] The preparation method of the cathode material in Comparative Example 2 is as follows: S1. Preparation of raw material dispersion: Add 100mL of pure water to a beaker, then add 12.538g of ammonium dihydrogen phosphate and stir to dissolve. Add 43.887g of ferric phosphate and 21.199g of sodium carbonate and stir vigorously. After the dispersion is uniform, add 3.049g of glucose and 3.049g of sucrose as carbon source and continue stirring for 15min to obtain a uniformly dispersed solution. S2. Coarse grinding and refining: The dispersion in S1 is placed in a grinding jar and loaded into a planetary ball mill. It is ball-milled at 500 rpm for 3 hours to obtain the first precursor. S3. The first precursor obtained in S2 is transferred to a sand mill and sand milled at 1200 rpm for 3 hours to obtain the second precursor; the second precursor is placed in a 100℃ oven and baked for 15 hours, and the dried powder is ground. S4. Preparation of cathode material: The powder obtained after grinding in S3 is placed in a ceramic boat and calcined at 550℃ under a N2 protective atmosphere with a heating rate of 5℃ / min for 10 hours to obtain an iron-based mixed phosphate cathode material (denoted as Na4Fe). 2.91 (PO4)2P2O7 / C).

[0058] Comparative Example 3 Comparative Example 3 provides a method for preparing a cathode material, which differs from Example 1 in that the molar ratio of Na:Fe:P in the Na source, Fe source, and phosphorus source is 4:3:4.

[0059] The preparation method of the cathode material in Comparative Example 3 is as follows: S1. Preparation of raw material dispersion: Add 100mL of pure water to a beaker, then add 11.503g of ammonium dihydrogen phosphate and stir to dissolve. Add 45.245g of ferric phosphate and 21.199g of sodium carbonate and stir vigorously until evenly dispersed. Then add 6.21g of glucose as carbon source and continue stirring for 15min to obtain a uniformly dispersed solution. S2. Coarse grinding and refining: The dispersion in S1 is placed in a grinding jar and put into a planetary ball mill and ball-milled at 450 rpm for 4 hours to obtain the first precursor; S3. Preparation of precursor materials: Graphene oxide was placed in deionized water and ultrasonically mixed to form a graphene oxide solution with a concentration of 5 mg / mL. 124 mL of graphene oxide solution was added to the first precursor, and then transferred to a sand mill and milled at 1500 rpm for 2.5 h to obtain the second precursor. The second precursor was placed in a 100℃ oven and baked for 15 h, and the dried powder was ground. S4. Preparation of cathode material: The powder obtained after grinding in S3 is placed in a ceramic boat and calcined at 600℃ at a rate of 3℃ / min under a N2 protective atmosphere for 10 hours to obtain a graphene-modified iron-based mixed phosphate cathode material (denoted as Na4Fe3(PO4)2P2O7 / C / rGO).

[0060] Comparative Example 4 Comparative Example 4 provides a method for preparing a cathode material, which differs from Example 1 in that it uses a spray drying method to prepare the cathode material. The specific preparation steps are as follows: S1. Preparation of raw material dispersion: Add 100mL of pure water to a beaker, then add 12.538g of ammonium dihydrogen phosphate and stir to dissolve. Add 43.887g of ferric phosphate and 21.199g of sodium carbonate and stir vigorously. After the dispersion is uniform, add 3.049g of glucose and 3.049g of sucrose. Finally, add 124mL of graphene oxide solution and stir continuously for 15min to obtain a uniformly dispersed dispersion. S2. Spray drying was used to granulate the dispersion obtained in S1. The inlet air temperature was controlled at 190℃, the feed rate was 7mL / min, and the spray atmosphere was air. Finally, the precursor dry powder was obtained. S3. Preparation of cathode material: The precursor powder obtained in S2 was placed in a ceramic boat and calcined at 550℃ under a N2 protective atmosphere at a rate of 5℃ / min for 10 hours to obtain a graphene-modified iron-based mixed phosphate cathode material (denoted as Na4Fe). 2.91 (PO4)2P2O7 / C / rGO).

[0061] Comparative Example 5 Comparative Example 5 provides a method for preparing a cathode material, which differs from Example 1 in that it only uses ball milling. The specific preparation method of the cathode material in Comparative Example 5 is as follows: S1. Preparation of raw material dispersion: Add 100mL of pure water to a beaker, then add 12.538g of ammonium dihydrogen phosphate and stir to dissolve. Add 43.887g of ferric phosphate and 21.199g of sodium carbonate and stir vigorously. After the dispersion is uniform, add 3.049g of citric acid and 3.049g of sucrose and continue stirring for 15min to obtain a uniformly dispersed solution. S2. Coarse grinding and refining: Graphene oxide is placed in deionized water to form an ultrasonic graphene oxide solution with a concentration of 5 mg / mL. 124 mL of graphene oxide solution is added to the dispersion obtained in S1 to obtain a mixture. The mixture is placed in a grinding jar and loaded into a planetary ball mill. The mixture is ball-milled at 400 rpm for 5 hours to obtain the first precursor. The first precursor is baked in a 100°C oven for 15 hours, and the dried powder is then ground. S3. Preparation of cathode material: The powder obtained after grinding in S2 is placed in a ceramic boat and calcined at 550℃ for 10 hours under a protective atmosphere of H2 / Ar (volume ratio of H2 to Ar is 5:95) at a rate of 4℃ / min. This yields a graphene-modified iron-based mixed phosphate cathode material (denoted as Na4Fe). 2.91 (PO4)2P2O7 / C / rGO).

[0062] Performance testing 1. Scanning electron microscopy observation The cathode material prepared in Example 1 was observed using a scanning electron microscope (SEM), and the SEM image is shown below. Figure 1 As shown.

[0063] Depend on Figure 1 The microstructure shows that the cathode material in Example 1 exhibits particle agglomeration and has an irregular porous block structure.

[0064] 2. Rate performance and cycle performance testing The half-cells were assembled using the positive electrode materials (active materials) prepared in Example 1 and Comparative Examples 1-5. The specific steps are as follows: The active material, conductive carbon black (Super P), and binder polyvinylidene fluoride (PVDF) are thoroughly mixed and dispersed in a mass ratio of 8:1:1 to form a uniform slurry. The resulting slurry is then uniformly coated onto aluminum foil and dried in a vacuum drying oven at 80°C. The resulting electrode is then cut into electrode sheets with a diameter of 10 mm for later use. Sodium metal and glass fiber membrane (GF / D) were used as the counter electrode and separator, respectively; NaPF6 and propylene carbonate (PC) system was used as the electrolyte with a concentration of 1.0 M; the prepared electrode sheet was used as the positive electrode, and the battery was assembled into a coin cell sodium-ion battery using an LIR2032 battery case.

[0065] The coin-type sodium-ion batteries prepared with the cathode materials of Examples 1 and 1-5 were tested at voltages ranging from 1.7 to 4.3 V and at temperatures ranging from 0.1 to 10 C (1 C = 129 mAh·g). -1 Rate performance tests were conducted at various levels. Table 1 shows the rate performance test results of the coin-type sodium-ion batteries prepared using the cathode materials of Examples 1-5 (Example 1). Table 1 also shows the rate performance test results of the coin-type sodium-ion batteries prepared using the cathode material of Comparative Example 1 (Example 1). Figure 2 As shown.

[0066] Table 1: Discharge specific capacity of coin cell sodium-ion batteries prepared with cathode materials from Examples 1-5 and Comparative Examples 1-5 (unit: / mAh·g) -1 )

[0067] As shown in Table 1, the coin cell sodium-ion battery prepared with the cathode material of Example 1 has a discharge specific capacity of 107.3 mAh·g at discharge rates of 0.1C, 0.2C, 0.5C, 1C, 2C, 5C, and 10C. -1 102.8mAh·g -1 100.1mAh·g -1 98.82mAh·g -1 98.1mAh·g -1 93.4mAh·g -1and 90.8mAh·g -1 It exhibits excellent rate performance.

[0068] From Table 1 and Figure 2 It can be seen that the coin cell sodium-ion battery prepared with the cathode material of Example 1 exhibits a performance of 98.82 mAh·g at 1C. -1 The discharge specific capacity was measured, and the calculated first-cycle coulombic efficiency was 98.52%. The coin cell sodium-ion battery obtained in Comparative Example 1 exhibited a discharge capacity of 84.9 mAh·g at 1C. -1 The discharge specific capacity and the first-cycle coulombic efficiency of 98.2% indicate that the rate performance of the coin cell sodium-ion battery prepared by the cathode material of Example 1 of this invention is better than that of Comparative Example 1.

[0069] In the cathode material of Comparative Example 1, no graphene coating layer or iron defects were provided, resulting in the coin cell sodium-ion battery of Comparative Example 1 having a lower discharge specific capacity than that of Example 1 at rates of 0.1C, 0.2C, 0.5C, 1C, 2C, 5C, and 10C. This indicates that the graphene coating layer and iron defects play an important role in improving the rate performance of the coin cell sodium-ion battery.

[0070] In Comparative Example 2, the cathode material lacked a graphene coating layer, resulting in a lower discharge specific capacity of the coin cell sodium-ion battery compared to Example 1 at rates of 0.1C, 0.2C, 0.5C, 1C, 2C, 5C, and 10C. This demonstrates that the graphene coating in the cathode material plays a crucial role in improving the rate performance of the coin cell sodium-ion battery.

[0071] In Comparative Example 3, no iron defects were present in the cathode material, resulting in a lower discharge specific capacity of the coin cell sodium-ion battery compared to Example 1 at rates of 0.1C, 0.2C, 0.5C, 1C, 2C, 5C, and 10C. This demonstrates that iron defects in the cathode material play an important role in improving the rate performance of the coin cell sodium-ion battery.

[0072] Comparative Example 4 uses a spray method to prepare the cathode material, resulting in a coin cell sodium-ion battery in Comparative Example 4 having a lower discharge specific capacity than Example 1 at rates of 0.1C, 0.2C, 0.5C, 1C, 2C, 5C, and 10C. This indicates that ball milling of the dispersions of Na, Fe, Phosphorus, and Carbon sources followed by sand milling of the system after adding graphene oxide solution plays an important role in improving the rate performance of the coin cell sodium-ion battery.

[0073] Comparative Example 5 directly ball-milled a mixture of dispersions of Na, Fe, Phosphorus, and Carbon sources, as well as a graphene oxide solution. This resulted in the coin cell sodium-ion battery of Comparative Example 5 having a lower discharge specific capacity than that of Example 1 at rates of 0.1C, 0.2C, 0.5C, 1C, 2C, 5C, and 10C. This demonstrates that the solid-phase synthesis method combining ball milling and sand milling plays an important role in improving the rate performance of coin cell sodium-ion batteries.

[0074] The coin-type sodium-ion batteries prepared with the cathode materials of Example 1 and Comparative Examples 1-5 operated at 5C (1C = 129 mAh·g) within a voltage range of 1.7-4.3V. -1 The discharge specific capacity after 2700 cycles was tested at a certain rate and is shown in Table 2.

[0075] Table 2: Discharge specific capacity of coin cell sodium-ion batteries prepared with cathode materials from Example 1 and Comparative Examples 1-5 after 2700 cycles

[0076] The cycle stability of the coin cell sodium-ion battery prepared with the cathode material of Example 1 and Comparative Example 1 is as follows: Figure 3 As shown.

[0077] From Table 2 and Figure 3 It can be seen that the cycling performance of Example 1 is significantly better than that of Comparative Examples 1-5, retaining 90.6 mAh·g after 2700 cycles. -1 The specific discharge capacity.

[0078] Comparative Examples 1-5 exhibited significantly worse cycle performance than Example 1 because they lacked a graphene coating, iron defects, or both, or did not employ a solid-state synthesis method combining ball milling and sand milling. This demonstrates that the synergistic effect of graphene coating and iron defects, as well as the solid-state synthesis method combining ball milling and sand milling, play a crucial role in improving battery cycle performance.

[0079] In summary, this invention, through the synergistic design of graphene coating technology and iron defects (Fe vacancies), simultaneously addresses three major bottlenecks in materials: low intrinsic electronic conductivity, insufficient active sites, and slow ion diffusion kinetics. This results in a breakthrough improvement in material performance, enabling the cathode material to maintain high operating voltage and structural stability while significantly improving room-temperature rate performance, and simultaneously enhancing cycle stability and reversible capacity. Furthermore, the solid-state synthesis method combining ball milling and sand milling can improve the electrochemical performance of coin cell sodium-ion batteries.

[0080] The above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit the scope of protection of the present invention. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can be made to the technical solutions of the present invention without departing from the essence and scope of the technical solutions of the present invention.

Claims

1. A positive electrode material, characterized by, The positive electrode material comprises a substrate and a coating layer. The chemical formula of the base is Na4Fe 3-3x (PO4)2P2O7, x is in the range of 0.01-0.06; The coating layer comprises an amorphous carbon layer and a graphene layer from the substrate side outward in turn.

2. The positive electrode material of claim 1, wherein, The carbon content in the positive electrode material is 4.5-11wt%.

3. The positive electrode material of claim 1, wherein, The positive electrode material has a porous structure with a pore size of 9-110nm; and / or, the particle size of the positive electrode material is 0.7-16μm.

4. The positive electrode material of claim 1, wherein, The graphene layer is a reduced graphene oxide layer.

5. The method of producing a positive electrode material according to any one of claims 1 to 4, characterized by, The preparation method comprises the following steps: (1) mixing an iron source, a sodium source, a phosphorus source, a carbon source and a solvent to obtain a dispersion liquid; the dispersion liquid is ball milled to obtain a first precursor; (2) mixing the first precursor and graphene, sand milling to obtain a second precursor; and then calcining to obtain the positive electrode material.

6. The preparation method according to claim 5, characterized in that, In step (1), the iron source comprises at least one of ferrous sulfate heptahydrate, ferric citrate, ferric oxalate, iron nitrate nonahydrate, iron sulfate, and iron phosphate; and / or, the sodium source comprises at least one of sodium carbonate, sodium pyrophosphate, sodium nitrate, sodium sulfate, sodium bicarbonate, sodium bicarbonate, and sodium dihydrogen phosphate; and / or, the phosphorus source comprises at least one of sodium dihydrogen phosphate, diammonium hydrogen phosphate, ammonium dihydrogen phosphate, and phosphoric acid; and / or, the carbon source comprises at least one of glucose, citric acid, sucrose, polyethylene glycol, and urea; and / or, the solvent comprises water.

7. The preparation method according to claim 5, characterized in that, In step (1), the molar ratio of the sodium source, the iron source and the phosphorus source is Na:Fe:P=4:(3-3x):4, x is in the range of 0.01-0.06; and / or, the addition amount of the carbon source is 4.5-11% of the theoretical product mass of the positive electrode material; and / or, the rotation speed of the ball milling is 320-550rpm, and the ball milling time is 3.5-6.5h.

8. The preparation method according to claim 5, characterized in that, In step (2), the graphene comprises graphene oxide; and / or, the addition amount of the graphene is 4.5-16.5% of the theoretical product mass of the positive electrode material.

9. The preparation method according to claim 5, characterized in that, In step (2), the rotation speed of the sand milling is 900-2200rpm, and the sand milling time is 1.8-5.5h; and / or, the calcination temperature is 350-650℃, and the calcination time is 6.5-12.5h.

10. A sodium-ion battery, characterized in that, The positive electrode material of any one of claims 1-4.

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