Multi-phase composite iron-based polyanion sodium ion battery positive electrode material and preparation method thereof

Abstract

The invention provides a multiphase composite iron-based polyanion sodium ion battery positive electrode material and a preparation method thereof, a planetary ball milling reinforced phase sintering method is adopted to regulate and control an initial raw material Na / Fe feeding ratio and F element loss in a high-temperature sintering process, and a series of multiphase materials with non-stoichiometric ratios are synthesized. Wherein the material with the optimal performance is a three-phase composite material of Na7Fe7 (PO4) 6F3, Na2FePO4F and Na2Fe3 (PO4) 3, and primary particles of the material have a heterojunction structure. Excellent sodium ion channels of Na2FePO4F and Na2Fe3 (PO4) 3 in the three phases are beneficial for the Na7Fe7 (PO4) 6F3 material to release high theoretical specific capacity, and the overall actual specific capacity of the material is improved; the low-strain stable lattice structure of the Na7Fe7 (PO4) 6F3 material can slow down the Na2FePO4F lattice strain, inhibit the phase change and maintain the structural integrity of the composite material in the long cycle process, so that the material has high capacity and excellent cycle performance.

Description

Technical Field

[0001] This invention belongs to the field of sodium-ion battery cathode materials, specifically relating to a novel multiphase composite cathode material and its preparation method. Background Technology

[0002] The widespread use of lithium-ion batteries is profoundly changing our lives: they not only power a wide variety of end products but also play a crucial role in the large-scale storage of green electricity. Faced with ever-increasing demand, the limitations and uneven distribution of lithium abundance are becoming increasingly prominent. Against this backdrop, sodium-ion batteries, with their similar working principles to lithium-ion batteries, comparable overall electrochemical performance, and the significant advantage of abundant and widely distributed sodium resources, have rapidly become a research hotspot in the energy storage field. They are widely considered by the industry to be an ideal complement to lithium-ion batteries in areas such as large-scale energy storage, and are expected to alleviate the problem of lithium resource dependence.

[0003] However, sodium ions and lithium ions differ significantly in their physicochemical properties, directly leading to substantial differences in the initial composition and structural evolution of sodium battery electrode materials compared to lithium battery electrode materials. Currently, the electrochemical performance of sodium-ion batteries remains somewhat insufficient, with the capacity performance of the cathode material being the core bottleneck. Taking the mainstream phosphate system as an example: the theoretical specific capacity of lithium-ion battery cathode material LiFePO4 is 170 mAh / g, with actual capacities typically exceeding 160 mAh / g and exhibiting excellent cycle stability; while the theoretical specific capacity of sodium-ion battery cathode material Na4Fe3(PO4)2P2O7 is only 129 mAh / g, and its actual discharge capacity is even lower than 120 mAh / g due to limitations in ion migration efficiency and structural stability. In a battery cell system, the specific capacity performance of the cathode material directly determines its energy density; therefore, insufficient cathode capacity has become a key factor restricting the improvement of sodium-ion battery energy density and hindering its commercialization. Based on this, developing sodium-ion battery cathode materials with high specific capacity, long cycle stability, and low cost is crucial for promoting the further commercialization of sodium-ion batteries. Summary of the Invention

[0004] To address the issue that current mainstream single-phase cathode materials for sodium-ion batteries (such as sodium iron pyrophosphate and sodium iron fluorinated phosphate) have low capacity and cannot meet the high energy density requirements of large-scale energy storage, this invention discloses a multiphase composite iron-based polyanion sodium-ion battery cathode material and its preparation method. By selecting low-cost raw materials and designing a simple preparation process to control production costs, and more importantly, by using a "multiphase coupling and synergy" mechanism to fully release the capacity of the Na7Fe7(PO4)6F3 phase in the composite material, the material as a whole has a discharge capacity far exceeding that of mainstream single-phase cathodes, promoting its industrial application in large-scale energy storage and other fields.

[0005] Specifically, the multiphase composite iron-based polyanion sodium-ion battery cathode material (hereinafter referred to as multiphase composite cathode material) provided by the present invention is composed of two or more phases selected from Na7Fe7(PO4)6F3, Na2FePO4F, Na2Fe3(PO4)3 and NaFePO4; preferably, the multiphase composite cathode material of the present invention is composed of three phases, Na7Fe7(PO4)6F3, Na2FePO4F and Na2Fe3(PO4)3, which can form heterojunctions with each other, thereby improving the capacity and cycle stability of the material through multiphase synergy.

[0006] Furthermore, all primary particles of the multiphase composite cathode material are coated with an amorphous carbon layer. For example, the heterojunction formed by the three phases Na7Fe7(PO4)6F3, Na2FePO4F, and Na2Fe3(PO4)3 together constitutes spherical primary particles coated with an amorphous carbon layer. The synergistic effect of the three phases is beneficial to improving electrochemical performance. The carbon layer can improve the conductivity of the material and reduce interfacial side reactions.

[0007] Furthermore, the apparent chemical formula of the multiphase composite cathode material provided by this invention is Na. 2-2x Fe 1+x PO4F (x<1). The apparent chemical formula only represents the overall atomic ratio of Na, Fe, P, and F in the material and does not correspond to the stoichiometry of a single crystal phase. The actual chemical formula of each crystal phase still follows its inherent structure. In the formula, the specific ratio of Na and Fe elements is determined by the initial feeding ratio of Na and Fe sources. The actual F element content of the material is lower than the theoretical value of 1 F atom per unit of apparent chemical formula. The specific value is determined by the initial F source feeding amount and the F element loss during high-temperature sintering.

[0008] Furthermore, the primary particle size morphology of the multiphase composite cathode material is nanoscale spherical.

[0009] This invention also provides a method for preparing the multiphase composite iron-based polyanion sodium-ion battery cathode material, comprising the following steps: (1) Sodium, iron, phosphorus and fluorine sources were placed in a ball mill jar and planetary ball milled using acetone as a grinding aid. After thorough mixing, a uniform slurry was obtained. The slurry was dried in an oven and then sieved using a vibrating sieve to obtain a uniform mixed raw material powder.

[0010] (2) The mixed raw material powder is placed in a tube furnace with an inert atmosphere and heated to 300-400°C at a heating rate of 1-3°C / min. The temperature is held for 1-3 hours to complete the first pre-calcination. The powder after the first pre-calcination is taken out, ground thoroughly, and then placed in a tube furnace with an inert atmosphere again. The temperature is heated to 300-400°C at a heating rate of 1-3°C / min and held for 1-3 hours to complete the second pre-calcination, and finally the pre-calcined material is obtained.

[0011] (3) Mix the pre-burned material with the carbon source thoroughly to obtain a uniform mixture.

[0012] (4) The mixture is pressed into sheets and placed in a tube furnace with an inert atmosphere. The temperature is first raised to 240-300℃ at a heating rate of 1-3℃ / min and held for sintering for 1-3 hours. Then the temperature is raised to 550-650℃ and held for sintering for 8-12 hours to obtain a multiphase composite iron-based polyanion sodium-ion battery cathode material. The sintering environment can be either sealed sintering or open sintering to control the F content.

[0013] Preferably, the sodium source is selected from one or more of sodium carbonate, sodium bicarbonate, and sodium fluoride. The iron source is selected from one or two of ferrous oxalate dihydrate and ferric phosphate. The phosphorus source is selected from one or two of ammonium dihydrogen phosphate and ferric phosphate. The fluorine source is selected from one or two of sodium fluoride and polytetrafluoroethylene. The carbon source is selected from one or more of polyvinyl alcohol, polytetrafluoroethylene, glucose, and sucrose, wherein the polyvinyl alcohol is preferably one or more of 200 mesh, 600 mesh, and 800 mesh.

[0014] In the above preparation process of the present invention: The first pre-calcination can completely remove residual acetone solvent from the system and promote the decomposition reaction of the iron source (such as the decomposition of ferrous oxalate dihydrate into FeO, FeC2O4·2H2O). The reaction process involves FeO + CO + H2O, simultaneously removing volatile gases such as CO generated during the reaction. Before the second pre-calcination, the hard agglomerated particles formed after the first pre-calcination are effectively broken up by grinding, fully exposing incompletely decomposed impurities (such as residual ammonium dihydrogen phosphate) encased within the particles. During the second pre-calcination process, these exposed impurities can be completely decomposed (NH4H2PO4). The process of pre-nucleation (NH3+H3PO4) avoids the sudden release of impurities during high-temperature sintering, which could cause the carbon coating layer to crack and ensure its integrity. Simultaneously, two low-temperature pre-firing processes induce the formation of uniformly sized quasi-nuclei (such as the initial crystal phase of Na2Fe3(PO4)3). This pre-nucleation mechanism effectively avoids the imbalance in multiphase distribution caused by rapid grain growth during direct high-temperature sintering, laying the microscopic foundation for the final formation of a uniform three-phase heterojunction structure.

[0015] After pre-firing, high-temperature sintering promotes the growth of quasi-crystal nuclei into the target crystalline phase and forms a heterojunction, while carbonizing the carbon source to form an amorphous carbon layer. Particle densification and fixation of nanoscale morphology can be achieved by using a tableting and subsequent sintering method; the F retention rate can be controlled by utilizing a sealed / open sintering environment to suppress impurity phases.

[0016] Compared with existing technologies, the beneficial effects of this invention are reflected in: 1. To address the issue of low actual capacity in existing single-phase iron-based polyanionic cathode materials, this invention constructs a multiphase composite system and utilizes a multiphase ion channel synergistic mechanism to fully release the electrochemical activity of the phase with high theoretical capacity. For example, in the three-phase heterojunction structure of Na7Fe7(PO4)6F3 / Na2FePO4F / Na2Fe3(PO4)3, the two-dimensional through-channel ion channel of Na2FePO4F and the one-dimensional linear ion channel of Na2Fe3(PO4)3 can form a three-dimensional ion transport network with some of the connected channels of Na7Fe7(PO4)6F3, significantly promoting the release of the capacity of the Na7Fe7(PO4)6F3 phase. This results in the composite heterojunction material exhibiting excellent actual specific capacity, meeting the high energy density requirements of large-scale energy storage.

[0017] 2. This invention solves the problems of large lattice strain and easy structural collapse during cycling of traditional single-phase cathode materials through the synergistic regulation of multiphase components. Na7Fe7(PO4)6F3 has a hexagonal stable lattice structure, and in Na... + The low volumetric strain rate during the insertion / extraction process allows its nanodomains to act as structural support units, limiting the lattice strain of Na2FePO4F during the electrochemical process, preventing the formation of intermediate phases during single-cycle operation, maintaining the integrity of the lattice structure of the composite material during long-cycle phase transition, and thus ensuring the integrity of the overall heterojunction structure. This enables the composite material to exhibit excellent cycle performance, meeting the core requirements of large-scale energy storage for battery life.

[0018] 3. This invention uses low-cost raw materials and adopts a conventional solid-state preparation process of "planetary ball milling + two-stage pre-calcination + one-time sintering". It does not require complicated sol-gel or hydrothermal equipment. While improving performance, it also takes into account the economy and repeatability of the preparation process, laying the foundation for industrial application. Attached Figure Description

[0019] Figure 1 The X-ray diffraction patterns are those of the multiphase composite cathode materials prepared in Examples 1-6 of this invention.

[0020] Figure 2 The images in the middle (a, b) are transmission electron microscope (TEM) images of the multiphase composite cathode materials prepared in Examples 1-6 of this invention.

[0021] Figure 3The images shown are transmission electron microscope (TEM) images of the multiphase composite cathode material prepared in Example 2 of this invention. In the images, a is a high-resolution TEM image of the primary particles, b is a magnified view of the three-phase heterojunction interface, and c, d and e are the fast Fourier transform diffraction spots and calibration results of the three crystal lattices, respectively, corresponding to the Na7Fe7(PO4)6F3, Na2FePO4F and Na2Fe3(PO4)3 phases.

[0022] Figure 4 The figures are the electrochemical curves and cycling curves of the single-phase cathode material prepared in the comparative examples of the present invention, wherein: a is the voltage curve of the sample in Comparative Example 1 at a current of 0.1C, b is the cycling curve of the sample in Comparative Example 1 at a current of 0.5C, c is the voltage curve of the sample in Comparative Example 2 at a current of 0.1C, and d is the cycling curve of the sample in Comparative Example 2 at a current of 0.5C.

[0023] Figure 5 The values ​​in a,b, and f represent the discharge curves of the multiphase composite cathode materials prepared in Examples 1-6 of this invention, respectively.

[0024] Figure 6 The values ​​in 'ac' represent the cycling curves of the multiphase composite cathode materials prepared in Examples 1, 2, and 6 of this invention, respectively. Detailed Implementation

[0025] The embodiments of the present invention are described in detail below. These embodiments are implemented based on the technical solution of the present invention, and provide detailed implementation methods and specific operation processes. However, the scope of protection of the present invention is not limited to the following embodiments.

[0026] Unless otherwise stated, the methods, reagents, and materials used in the following examples are conventional in the art. All raw material compounds used in the examples are commercially available.

[0027] Example 1 In this embodiment, a multiphase composite cathode material is prepared according to the following steps: (1) 1.2597 g NaF (0.03 mol), 3.4509 g NH4H2PO4 (0.03 mol), 1.8902 g NaHCO3 (0.0225 mol), 6.3415 g FeC2O4·2H2O (0.03525 mol) and 30 mL acetone were added to a 250 mL ball mill jar, and the jar was placed in a planetary ball mill for wet ball milling. The overall ball milling process consisted of three stages: 600 rpm for 0.5 hours, 500 rpm for 1 hour, and finally 400 rpm for 10 hours. This multi-step process facilitated the thorough crushing, grinding, and mixing of the raw materials. The resulting slurry was then dried at 80°C and sieved using a 300-mesh standard sieve to obtain a uniform, light yellow mixed raw material powder.

[0028] (2) Spread 5 g of mixed raw material powder evenly in an alumina boat, place it in a tube furnace with argon atmosphere, heat it to 350°C at a heating rate of 2°C / min, and hold it for 3 hours to complete the first pre-calcination. Take out the powder after the first pre-calcination, put it in an agate mortar and grind it thoroughly, and put it in the tube furnace again to repeat the above pre-calcination process (under the same conditions) to obtain the pre-calcined material.

[0029] (3) Grind and mix the pre-burned material with 15 wt% of polyvinyl alcohol powder (degree of polymerization of 1700, degree of hydrolysis of 88%, particle size of 200 mesh) in an agate mortar to obtain a uniform mixture.

[0030] (4) Using a mold with a diameter of 1 cm, the mixture was pressed into discs under a pressure of 3 MPa, placed in an open alumina boat, and then placed in a tube furnace with an argon atmosphere. The temperature was increased to 260°C at a heating rate of 2°C / min and held for 2 hours. Then, the temperature was increased to 600°C at a heating rate of 2°C / min and held for 10 hours to obtain uniformly carbon-coated Na. 1.75 Fe 1.125 The XRD pattern of the PO4F sample is shown in the figure. Figure 1 It consists of three phases: Na7Fe7(PO4)6F3, Na2FePO4F, and Na2Fe3(PO4)3. 1.75 Fe 1.125 Transmission electron microscopy (TEM) images of the PO4F sample are shown below. Figure 2 a. Primary particles are nanoscale spherical particles.

[0031] Example 2 In this embodiment, the multiphase composite cathode material was prepared according to the same process steps and conditions as in Example 1. The only difference is that in step (1), the raw materials are 1.2597 g NaF (0.03 mol), 3.4509 g NH4H2PO4 (0.03 mol), 1.2601 g NaHCO3 (0.015 mol), and 6.7463 g FeC2O4·2H2O (0.0375 mol).

[0032] The product obtained in this embodiment is Na with uniform carbon coating. 1.5 Fe 1.25 The XRD pattern of the PO4F sample is shown in the figure. Figure 1 It consists of three phases: Na7Fe7(PO4)6F3, Na2FePO4F, and Na2Fe3(PO4)3. 1.5 Fe 1.25 Transmission electron microscopy (TEM) images of the PO4F sample are shown below. Figure 2 b, the primary particles are nanoscale spherical particles.

[0033] Example 3 In this embodiment, the multiphase composite cathode material was prepared according to the same process steps and conditions as in Example 1. The only difference is that in step (1), the raw materials are 1.2597 g NaF (0.03 mol), 3.4509 g NH4H2PO4 (0.03 mol), 0.9451 g NaHCO3 (0.01125 mol), and 7.0836 g FeC2O4·2H2O (0.039375 mol).

[0034] The product obtained in this embodiment is Na with uniform carbon coating. 1.375 Fe 1.3125 The XRD pattern of the PO4F sample is shown in the figure. Figure 1 It consists of four phases: Na7Fe7(PO4)6F3, Na2FePO4F, Na2Fe3(PO4)3, and NaFePO4. 1.375 Fe 1.3125 Transmission electron microscopy (TEM) images of the PO4F sample are shown below. Figure 2 c, the primary particles are nanoscale spherical particles.

[0035] Example 4 In this embodiment, the multiphase composite cathode material was prepared according to the same process steps and conditions as in Example 1. The only difference is that in step (1), the raw materials are 1.2597 g NaF (0.03 mol), 3.4509 g NH4H2PO4 (0.03 mol), 0.8401 g NaHCO3 (0.01 mol), and 7.196 g FeC2O4·2H2O (0.04 mol).

[0036] The product obtained in this embodiment is Na with uniform carbon coating. 4 / 3 Fe 4 / 3 The XRD pattern of the PO4F sample is shown in the figure. Figure 1 It consists of four phases: Na7Fe7(PO4)6F3, Na2FePO4F, Na2Fe3(PO4)3, and NaFePO4. 4 / 3 Fe 4 / 3 Transmission electron microscopy (TEM) images of the PO4F sample are shown below. Figure 2 d, the primary particles are nanoscale spherical particles or square submicron particles.

[0037] Example 5 In this embodiment, the multiphase composite cathode material was prepared according to the same process steps and conditions as in Example 1. The only difference is that in step (1), the raw materials are 1.2597 g NaF (0.03 mol), 3.4509 g NH4H2PO4 (0.03 mol), 0.6301 g NaHCO3 (0.0075 mol), and 7.4209 g FeC2O4·2H2O (0.04125 mol).

[0038] The product obtained in this embodiment is Na with uniform carbon coating. 1.25 Fe 1.375 The XRD pattern of the PO4F sample is shown in the figure. Figure 1 It consists of three phases: Na7Fe7(PO4)6F3, Na2FePO4F, and NaFePO4. 1.25 Fe 1.375 Transmission electron microscopy (TEM) images of the PO4F sample are shown below. Figure 2 e, the primary particles are nanoscale spherical particles or square submicron particles.

[0039] Example 6 In this embodiment, a multiphase composite cathode material is prepared using the same feed ratio and process conditions as in Example 4. The only difference is that a closed alumina boat is used for sintering in step (4) to suppress F loss.

[0040] The product obtained in this embodiment is F-Na with uniform carbon coating. 4 / 3 Fe 4 / 3 The XRD pattern of the PO4F sample is shown in the figure. Figure 1 It consists of three phases: Na7Fe7(PO4)6F3, Na2FePO4F, and Na2Fe3(PO4)3. 4 / 3 Fe 4 / 3 Transmission electron microscopy (TEM) images of the PO4F sample are shown below. Figure 2 f represents primary particles that are nanoscale spherical particles. Figure 3 a is Na 4 / 3 Fe 4 / 3High-resolution transmission image of a PO4F sample. (By...) Figure 3 b clearly shows the heterojunction interface formed by the three phases sharing a crystal lattice. Figure 3 c, d, and e show the fast Fourier transform diffraction spots of the two lattice structures. Calibration confirms that the three lattices correspond to the Na7Fe7(PO4)6F3, Na2FePO4F, and Na2Fe3(PO4)3 phases, respectively.

[0041] Comparative Example 1 This comparative example prepared a single-phase sample using the same process steps and conditions as Example 1, with the difference being that: in step (1), the raw materials were 6.2965 g FeC2O4·2H2O (0.035 mol), 3.4509 g NH4H2PO4 (0.03 mol), 0.6299 g NaF (0.015 mol), and 1.6802 g NaHCO3 (0.02 mol). In step (3), the carbon source was 8 wt% polyvinylidene fluoride (PVDF, Solef 5130, Solvay, particle size 600 mesh). The sample obtained in this comparative example was a uniformly carbon-coated Na7Fe7(PO4)6F3 single-phase sample.

[0042] Comparative Example 2 This comparative example prepared a single-phase sample using the same process steps and conditions as Example 1, with the difference being that the raw materials in step (1) were 3.598 g FeC2O4·2H2O (0.02 mol), 2.3006 g NH4H2PO4 (0.02 mol), 0.9238 g NaF (0.022 mol), and 1.5122 g NaHCO3 (0.018 mol). It is worth noting that, considering the volatilization loss of F element during high-temperature sintering, NaF with a stoichiometric excess of 10% was used. At the same time, in order to maintain the Na element content in accordance with the stoichiometric ratio, NaHCO3 was reduced by 10%. The sample obtained in this comparative example was a uniformly carbon-coated Na2FePO4F single-phase sample.

[0043] Comparing the above examples and comparative examples, it can be seen that: the samples prepared in Examples 1, 2, and 6 are three-phase composite materials of Na7Fe7(PO4)6F3, Na2FePO4F, and Na2Fe3(PO4)3; the samples prepared in Examples 3 and 4 are four-phase composite materials of Na7Fe7(PO4)6F3, Na2FePO4F, Na2Fe3(PO4)3, and NaFePO4; and the material prepared in Example 5 is a three-phase composite material of Na7Fe7(PO4)6F3, Na2FePO4F, and NaFePO4. Examples 1-5 used the same synthesis conditions, only differing in the Na and Fe element feeding ratio. Furthermore, as the Na and Fe feeding ratio decreased, the phase composition of the samples underwent a regular change. That is, with the decrease of the Na and Fe feeding ratio, the NaFePO4 phase gradually appeared in the samples, and its content increased with the decrease of the feeding ratio. This indicates that a decrease in the Na and Fe element feeding ratio promotes the volatilization of F element during the high-temperature sintering process. In Examples 6 and 4, the feeding ratio of each element is the same, but the phase composition of Example 6 is three-phase while that of Example 4 is four-phase. This indicates that sintering the sample in a closed environment can significantly reduce the volatilization of F element during the process, thereby effectively suppressing the formation of NaFePO4 phase.

[0044] The electrochemical performance of the samples obtained in each embodiment and comparative example was tested using the following methods: The positive electrode sheet was prepared by the doctor blade casting method: First, 70 wt% of active material (ground into powder using an agate mortar), 20 wt% of acetylene black, and 10 wt% of PVDF (PVDF, Solef 5130, Solvay, dissolved in NMP to prepare a 5 wt% solution beforehand) and an appropriate amount of N-methylpyrrolidone (NMP, 99.9%, Aladdin) were mixed and placed in a DAC 150.1 FVZ-K mixer. After stirring at 2500 rpm for 30 min, the resulting uniform slurry was poured onto a flat aluminum foil current collector and coated with a 200 μm doctor blade. Then, the aluminum foil was dried in a vacuum oven at 80 ℃ for 12 hours. After the NMP has completely evaporated, the electrode sheets are sandwiched between sulfuric acid paper and compacted three times using a roller press. They are then punched into 1.2 cm diameter electrode discs using a cutting machine. The weight is recorded, and the discs are placed in a 65 °C vacuum drying oven for at least 4 hours before being transferred to a glove box for later use. The surface loading of the active material in the electrode is approximately 2 mg / cm². -2 .

[0045] Battery Assembly: Electrochemical tests were conducted using CR2032 coin cells. Sodium foil was used as both the counter and reference electrodes, and a Whatman glass fiber membrane was used as the separator. The electrolyte was a 1 mol / L solution. -1A sodium hexafluorophosphate (NaPF6) propylene carbonate (PC) solution was prepared, with 5 vol% fluoroethylene carbonate (FEC) added. All battery assembly processes were performed in an argon-filled glove box (Brown, MB-Unilab Pro SP, water / oxygen content <0.1 ppm). Constant current charge-discharge tests were conducted using a BTS-610 multichannel battery testing system.

[0046] Constant current charge-discharge tests were conducted by assembling a sodium-ion half-cell, and the results are as follows: Figure 4 a shows the voltage curve of the Na7Fe7(PO4)6F3 single-phase sample in Comparative Example 1 at a current of 0.1C. The actual capacity of this sample is only 86 mAh / g, which is far lower than its theoretical capacity of 159 mAh / g. Figure 4 b shows the cycling curve of the Na7Fe7(PO4)6F3 single-phase sample in Comparative Example 1 at a current of 0.5C. The capacity did not decrease significantly after 900 cycles, demonstrating excellent cycling stability. Figure 4 c is the voltage curve of the Na2FePO4F single-phase sample in Comparative Example 2 at a current of 0.1C. The actual capacity of this sample is 118 mAh / g, which is close to the theoretical capacity of 124 mAh / g of the material. Figure 4 d is the cycling curve of the Na2FePO4F single-phase sample in Comparative Example 2 at a current of 0.5C. Its cycling stability is poor, and the capacity retention rate is only 50% after 600 cycles.

[0047] Figure 5 a represents Na in Example 1 1.75 Fe 1.125 The discharge curve of the PO4F sample at a current of 0.1C shows a capacity of 130 mAh / g. Figure 5 b represents Na in Example 2 1.5 Fe 1.25 The discharge curve of the PO4F sample at a current of 0.1C shows a capacity of 135 mAh / g. Figure 5 c represents Na in Example 3 1.375 Fe 1.3125 The discharge curve of the PO4F sample at a current of 0.1C, with a capacity of 100 mAh / g; Figure 5 d represents Na in Example 4 4 / 3 Fe 4 / 3 The discharge curve of the PO4F sample at a current of 0.1C shows a capacity of 92 mAh / g. Figure 5 e represents Na in Example 5 1.25 Fe 1.375 The discharge curve of the PO4F sample at a current of 0.1C shows a capacity of 51 mAh / g. Figure 5 f represents F-Na in Example 64 / 3 Fe 4 / 3 The discharge curve of the PO4F sample at 0.1C showed a capacity of 120 mAh / g. Samples with a three-phase composition of Na7Fe7(PO4)6F3, Na2FePO4F, and Na2Fe3(PO4)3 (Examples 1, 2, and 6) all exhibited excellent capacity performance. Among them, the Na2Fe3F sample prepared in Example 2... 1.5 Fe 1.25 The PO4F sample exhibits a discharge capacity of 135 mAh / g, corresponding to a discharge energy density of 380 Wh / kg, which is exceptionally high among iron-based sodium phosphate battery cathode materials. Comparison of the capacity performance of samples from various examples demonstrates that the formation of the NaFePO4 phase in the multiphase composite material is detrimental to its capacity performance. Furthermore, when the composite material comprises three phases—Na7Fe7(PO4)6F3, Na2FePO4F, and Na2Fe3(PO4)3—the ion-channel-rich Na2FePO4F and Na2Fe3(PO4)3 can provide additional ion channels for sodium ions in Na7Fe7(PO4)6F3, significantly promoting the release of Na7Fe7(PO4)6F3 capacity and thus significantly improving the overall capacity performance of the composite multiphase material.

[0048] Figure 6 a represents Na in Example 1 1.75 Fe 1.125 Cycling curve of PO4F sample at 0.5C current, capacity retention is 66% after 900 cycles; Figure 6 b represents Na in Example 2 1.5 Fe 1.25 Cycling curve of PO4F sample at 0.5C current, capacity retention is 88% after 1200 cycles; Figure 6 c represents F-Na in Example 6 4 / 3 Fe 4 / 3 The cycling curve of the PO4F sample at 0.5C showed no significant capacity decay after 1200 cycles. The cycling stability of the multiphase composite material was significantly better than that of the single-phase Na2FePO4F material in Comparative Example 2. This indicates that the multiphase composite significantly improves the overall cycling stability of the material. Furthermore, increasing the content of the Na7Fe7(PO4)6F3 phase in the multiphase composite material can significantly improve the cycling stability of the material.

[0049] The above are merely exemplary embodiments of the present invention and are not intended to limit the present invention. The scope of protection of the present invention is defined by the claims. Any equivalent substitutions or improvements made to the raw material ratio, preparation process parameters, phase composition control methods, etc., based on the technical concept of the present invention should be included in the scope of protection of the present invention. At the same time, those skilled in the art can adjust the process details within the technical framework disclosed in the present invention according to actual application needs to achieve targeted optimization of material properties, and such adjustments also fall within the scope of protection of the present invention.

Claims

1. A multiphase composite iron-based polyanion sodium-ion battery cathode material, characterized in that, The cathode material is composed of two or more phases selected from Na7Fe7(PO4)6F3, Na2FePO4F, Na2Fe3(PO4)3 and NaFePO4.

2. The multiphase composite iron-based polyanion sodium-ion battery cathode material according to claim 1, characterized in that, The primary particles of the cathode material are all coated with an amorphous carbon layer.

3. The multiphase composite iron-based polyanion sodium-ion battery cathode material according to claim 1 or 2, characterized in that, The cathode material is composed of three phases: Na7Fe7(PO4)6F3, Na2FePO4F, and Na2Fe3(PO4)3. The three phases form a heterojunction and together constitute a spherical primary particle covered by an amorphous carbon layer.

4. The multiphase composite iron-based polyanion sodium-ion battery cathode material according to claim 1, characterized in that, The apparent chemical formula of the cathode material is Na. 2-2x Fe 1+x PO4F, x<1, where the specific ratio of Na and Fe elements is determined by the initial feed ratio of Na and Fe sources.

5. The multiphase composite iron-based polyanion sodium-ion battery cathode material according to claim 4, characterized in that, The actual F element content in the cathode material is lower than the theoretical value of 1 F atom per unit of apparent chemical formula. The specific value is determined by the initial F source feed amount and the amount of F element loss during the high-temperature sintering process.

6. The multiphase composite iron-based polyanion sodium-ion battery cathode material according to claim 1, characterized in that, The primary particle size morphology of the cathode material is nanoscale spherical.

7. A method for preparing the multiphase composite iron-based polyanion sodium-ion battery cathode material according to any one of claims 1-6, characterized in that, Includes the following steps: (1) Mix the sodium source, iron source, phosphorus source and fluorine source evenly to obtain a mixed raw material powder; (2) The mixed raw material powder is kept at 300-400℃ for 1-3 hours in an inert atmosphere to complete the first pre-firing; the powder after the first pre-firing is taken out, ground thoroughly, and then kept at 300-400℃ for 1-3 hours in an inert atmosphere to complete the second pre-firing, and finally the pre-firing material is obtained. (3) Thoroughly mix the pre-burned material with the carbon source to obtain a uniform mixture; (4) Press the mixture into sheets, and in an inert atmosphere, first heat to 240-300℃ and hold for sintering for 1-3 hours, then continue to heat to 550-650℃ and hold for sintering for 8-12 hours to obtain multiphase composite iron-based polyanion sodium ion battery cathode material.

8. The preparation method according to claim 7, characterized in that: The sodium source is selected from one or more of sodium carbonate, sodium bicarbonate, and sodium fluoride; the iron source is selected from one or two of ferrous oxalate dihydrate and ferric phosphate; the phosphorus source is selected from one or two of ammonium dihydrogen phosphate and ferric phosphate; the fluorine source is selected from one or two of sodium fluoride and polytetrafluoroethylene; and the carbon source is selected from one or more of polyvinyl alcohol, polytetrafluoroethylene, glucose, and sucrose.

9. The preparation method according to claim 7, characterized in that: In step (1), a planetary ball mill is used to mix the materials; in steps (2) and (4), a tubular atmosphere furnace is used for heat preservation, with a heating rate of 1-3℃ / min.

10. The preparation method according to claim 7, characterized in that: In step (4), the sintering environment of the mixture is either sealed sintering or open sintering.

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