Sodium ferric phosphate pyrophosphate composite material, and preparation process, evaluation method and process optimization method therefor and use thereof

Abstract

The present invention relates to a sodium-ion battery material, and specifically relates to a sodium ferric phosphate pyrophosphate composite material, and a preparation process, evaluation method and process optimization method therefor and the use thereof. The sodium ferric phosphate pyrophosphate composite material has a spinel-type NaFePO4 phase; and in an XRD diffraction pattern obtained by subjecting the sodium ferric phosphate pyrophosphate composite material to XRD diffraction using CuKα radiation, there are a characteristic diffraction peak T of sodium ferric phosphate pyrophosphate and a characteristic diffraction peak O of spinel-type NaFePO4 at 2θ of 33.6±0.2° and 32.8±0.2°, respectively, the peak intensities thereof are respectively IT and IO, and the diffraction peak intensity ratio Ir is equal to IT / IO, wherein 6.19≤Ir≤13.

Description

Sodium iron pyrophosphate composite material and its preparation process, evaluation method, process optimization method and application

[0001] This application claims priority to the Chinese patent application filed with the China Patent Office on November 29, 2023, with application number 202311612427.2 and application name “Sodium ferric pyrophosphate composite material and its preparation process, evaluation method, process optimization method and application”, the entire contents of which are incorporated by reference into this application. Technical Field

[0002] The present application belongs to the technical field of sodium ion battery materials, and specifically relates to a sodium iron pyrophosphate composite material and its preparation process, evaluation method, process optimization method and application. Background Art

[0003] Sodium-ion batteries are rocking-chair batteries, a type of secondary battery that relies on the reciprocal insertion and extraction of ions between the positive and negative electrodes. Both the positive and negative electrode materials allow for the reversible insertion and extraction of sodium ions. Research on sodium-ion batteries began in 1982 and has made significant progress in recent years, bringing them into commercial use. Currently, the main cathode materials for sodium-ion batteries include oxides, Prussian blue, and polyanions.

[0004] As a polyanion sodium ion battery cathode material, sodium iron pyrophosphate composite material has a structurally stable sodium ion diffusion channel and good cycle performance, as well as moderate operating voltage and specific capacity. It is a cheap and promising sodium ion battery cathode material.

[0005] Currently, the most common synthesis method for this material is the liquid phase method, which has high requirements for reaction process conditions, complex production equipment, and difficulty in mass production, making it unsuitable for large-scale industrial applications. In addition, there is also a scheme for preparing sodium ferric pyrophosphate material by wet homogeneous sand milling combined with spray drying. However, in order to ensure the acquisition of pure phase Na4Fe3(PO4)2(P2O7), it is necessary to achieve uniform dispersion of the precursor salt at the nanoscale and control the particle size between 1 and 0.1 μm, which places high requirements on the operating process and reaction equipment. In addition, the capacity performance of sodium ferric pyrophosphate composite materials obtained by the existing synthesis process is limited, and there is still room for further improvement.

[0006] Summary of the Invention

[0007] In response to the above technical problems, one of the objectives of this application is to provide a sodium iron pyrophosphate composite material.

[0008] The second purpose of this application is to provide a preparation process of sodium iron pyrophosphate composite material.

[0009] The third purpose of this application is to provide an evaluation method for sodium iron pyrophosphate composite materials.

[0010] The fourth purpose of this application is to provide an optimization method for the preparation process of sodium iron pyrophosphate composite material.

[0011] The fifth object of this application is to provide a sodium iron pyrophosphate composite material for use as a positive electrode material in sodium ion batteries.

[0012] Compared with the prior art that pursues the acquisition of pure phase Na4Fe3(PO4)2(P2O7), the present application creatively discovered that when the sodium iron pyrophosphate composite material has a specific content of spinel-type NaFePO4 phase, the capacity performance of the sodium iron pyrophosphate composite material can be effectively improved. However, the NaFePO4 phase in the sodium iron pyrophosphate composite material may also play a certain structural phase role. When its content is too low, the structural stability of the material will be destroyed, thereby reducing the overall electrochemical performance; and when its content is too high, the overall electrochemical performance will be reduced.

[0013] Thus, the present application provides a sodium iron pyrophosphate composite material, wherein the sodium iron pyrophosphate composite material has a spinel NaFePO4 phase; the sodium iron pyrophosphate composite material is subjected to XRD diffraction under CuKα radiation, and the obtained XRD diffraction pattern has a characteristic diffraction peak T of sodium iron pyrophosphate and a characteristic diffraction peak O of spinel NaFePO4 at 2θ of 33.6±0.2° and 32.8±0.2°, and the peak intensities thereof are respectively I T and I O , let the diffraction peak intensity ratio Ir=I T / I O , where 6.19≤Ir≤13.

[0014] Furthermore, the sodium ferric pyrophosphate composite material comprises sodium ferric pyrophosphate and a carbon layer coated on the surface of the sodium ferric pyrophosphate, and the chemical formula is Na4Fe 3(1-x) (PO4)2P2O7@C, where 0≤x≤0.02.

[0015] Further preferably, the carbon content of the sodium iron pyrophosphate composite material is 2 to 5 wt%.

[0016] Further preferably, the size of the primary particles of the sodium iron pyrophosphate composite material is mainly distributed in the range of 100 to 200 nm, and the secondary particles are formed by agglomeration of the primary particles, and the size is mainly distributed in the range of 300 to 800 nm.

[0017] Based on the influence of NaFePO4 phase content on material properties, NaFePO4 phase control is the most core difficulty in the synthesis of this material.

[0018] In this regard, the present application also provides a preparation process of the sodium iron pyrophosphate composite material, comprising the following steps:

[0019] (1) Weigh the synthesized Na4Fe according to the preset feeding ratio 3(1-x) The raw materials of (PO4)2P2O7 and an appropriate amount of carbon source are wet-grinded and mixed to obtain a mixture;

[0020] (2) After the mixture is dried, it is sintered in a protective atmosphere according to the preset sintering parameters to obtain the sodium iron pyrophosphate composite material Na4Fe 3(1-x) (PO4)2P2O7@C;

[0021] Among them, 0≤x≤0.02.

[0022] Furthermore, in step (1): the synthesis of Na4Fe 3(1-x) The raw materials of (PO4)2P2O7 include an iron source and a sodium source.

[0023] Further preferably, the iron source is ammonium ferrous phosphate, and the sodium source includes sodium pyrophosphate and a P-free sodium source.

[0024] Further preferably, the P-free sodium source is selected from any one or more of sodium carbonate, ammonium bicarbonate, sodium acetate and sodium citrate.

[0025] More preferably, the molar ratio of ammonium ferrous phosphate, the sodium source not containing P and sodium pyrophosphate is 3: (0.96-1): (0.5-0.54).

[0026] Furthermore, the carbon source is any one or more of glucose, sucrose, starch, and polyethylene glycol.

[0027] More preferably, the amount of the carbon source is Na4Fe 3(1-x) 5 to 20% of the mass of the raw material of (PO4)2P2O7.

[0028] Furthermore, the wet grinding and mixing time is 4 to 6 hours; the medium of the wet grinding is a solvent with a boiling point lower than 80° C., such as ethanol.

[0029] Furthermore, in step (1), the grinding method is vibration milling or rolling milling, and more preferably continuous rolling ball milling.

[0030] Furthermore, in step (2), the preset sintering parameters include the sintering heating rate, the sintering temperature and the sintering time, which meet at least one of the following conditions:

[0031] The sintering heating rate is 10-15°C / min;

[0032] The sintering temperature is 500-550°C;

[0033] The sintering time is 8 to 12 hours.

[0034] Furthermore, in step (2), the protective atmosphere is an inert gas or nitrogen; more preferably, it is one of high-purity nitrogen, argon, and argon-hydrogen mixed gas.

[0035] The present application discloses a method for evaluating a sodium iron pyrophosphate composite material, comprising the following steps:

[0036] S1. The sodium iron pyrophosphate composite material is subjected to an XRD diffraction test under CuKα radiation, and an XRD diffraction pattern is obtained. According to the peak intensity of the required characteristic diffraction peak, the corresponding performance test value is calculated;

[0037] S2. Obtain performance evaluation results by comparing the performance test values ​​with the performance reference values.

[0038] Furthermore,

[0039] The performance test value in step S1 is Ir', Ir'=I T ' / I O ', where I T ' is the peak intensity of the characteristic diffraction peak T at 2θ of 33.6±0.2° in the XRD diffraction pattern, I O ' is the peak intensity of the characteristic diffraction peak O at 2θ of 32.8±0.2° in the XRD diffraction pattern;

[0040] The performance reference value in step S2 is Ir, and its interval range is 6.19≤Ir≤13; when Ir' is within the interval range of Ir, the performance evaluation result of the material is excellent.

[0041] The present study found that there are two possible reaction pathways in the synthesis reaction of sodium iron pyrophosphate composite material, namely: 4NH4FePO4·H2O+Na2CO3+Na4P2O7→2NaFePO4+2Na2FeP2O7+CO2+4NH3+6H2O (1) 6NH4FePO4·H2O+2Na2CO3+Na4P2O7→2Na4Fe3(PO4)2(P2O7)+6NH3+9H2O+2CO2 (2)

[0042] During the reaction process, both reactions may occur and compete with each other. However, since reaction (1) has a greater entropy increase and a greater thermodynamic advantage, and the phase formation temperature range of NaFePO4 is approximately 300-400°C, the electrochemically inert NaFePO4 has a stronger tendency to form the target product than reaction (2).

[0043] Based on the above findings, the present application can reduce the phase formation time of NaFePO4 by increasing the heating rate, and combine it with the method of using lattice defects (controlling the iron-phosphorus ratio) to inhibit the formation of thermodynamically dominant impure phases to reduce the content of inert phases, thereby improving the material capacity performance.

[0044] In this regard, the present application discloses a method for optimizing the preparation process of a sodium iron pyrophosphate composite material, which uses the above-mentioned sodium iron pyrophosphate composite material evaluation method to evaluate the sodium iron pyrophosphate composite material prepared by the above-mentioned preparation process of the composite sodium iron phosphate positive electrode material, and further includes the following steps:

[0045] S3. By comparing the performance test value and the performance reference value, it is determined whether the performance test value is within the range of the performance reference value;

[0046] If the performance test value is not within the range of the performance reference value, repeat steps (1), (2), S1, and S2, and optimize and adjust the process parameters in step (1) or (2) according to the comparison results until the performance test value is within the range of the performance reference value, and then output the optimized process parameters.

[0047] Furthermore, the process parameters in step (1) or (2) are optimized and adjusted according to the comparison results, wherein the process parameters include the amount of iron source added to the preset feed in step (1) and / or the preset sintering parameters in step (2).

[0048] Further preferably, when Ir' is less than the lower limit of the interval of Ir, the amount of iron source added is correspondingly reduced and / or the heating rate is increased; when Ir' is greater than the upper limit of the interval of Ir, the amount of iron source added is correspondingly increased and / or the heating rate is reduced.

[0049] The present application also discloses the use of the above-mentioned sodium iron pyrophosphate composite material, or the sodium iron pyrophosphate composite material prepared by the above-mentioned preparation process, or the sodium iron pyrophosphate composite material evaluated as excellent by the above-mentioned evaluation method, or the sodium iron pyrophosphate composite material prepared by the sodium iron pyrophosphate composite material preparation process optimized by the above-mentioned optimization method as a positive electrode material in a sodium ion battery.

[0050] This application has the following beneficial effects:

[0051] (1) In the prior art, attempts are generally made to obtain pure phase Na4Fe3(PO4)2(P2O7) through process improvements, and to reduce the electrochemically inert spinel-type NaFePO4 phase to avoid its influence on the electrochemical performance of the material. The present application creatively discovered that when a sodium ferric pyrophosphate composite material has a specific content of spinel-type NaFePO4 phase, the capacity performance of the material can be effectively improved. Experiments have shown that the sodium ferric pyrophosphate composite material with a specific content of NaFePO4 has an average capacity that is about 10% higher than that of materials with lower or higher content.

[0052] (2) The present application limits the content of the spinel-type NaFePO4 phase by the ratio of the characteristic peak and its peak intensity in the XRD spectrum obtained by XRD diffraction of the sodium iron pyrophosphate composite material under CuKα radiation, while ensuring the improvement of the capacity performance and avoiding the influence of its electrochemical inertness on other electrochemical properties of the material.

[0053] (3) This application proposes a solid-phase synthesis method for composite sodium iron phosphate based on NH4FePO4·H2O as the iron source. The raw material cost is low, the formula is easy to control, the synthesized product NaFePO4 (Marticite) has an appropriate content, the process is simple, the economic benefit is high, and the prepared material has excellent electrochemical properties.

[0054] (4) The present application uses an evaluation method for sodium iron phosphate pyrophosphate composite materials to evaluate the performance of the prepared composite sodium iron phosphate. When the evaluation fails, the process is adjusted to control the iron-phosphorus ratio and sintering temperature, reduce the residence time in the NaFePO4 phase formation temperature range, control the phase formation of the NaFePO4 phase, and then control the content of the NaFePO4 phase in the product, thereby obtaining a process for preparing products with better performance. BRIEF DESCRIPTION OF THE DRAWINGS

[0055] In order to more clearly illustrate the embodiments of the present application or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the embodiments or the description of the prior art. Obviously, the drawings described below are some embodiments of the present application. For ordinary technicians in this field, other drawings can be obtained based on these drawings without any creative work.

[0056] FIG1 is an XRD pattern of the sodium iron pyrophosphate composite material prepared in Example 1.

[0057] FIG2 is an SEM image of the sodium iron pyrophosphate composite material prepared in Example 1.

[0058] [Corrected 13.12.2024 according to Rule 91] Figure 3-1 is the charge and discharge curve of the button battery assembled with the sodium iron pyrophosphate composite material prepared in Example 1. [0058.1] [Corrected 13.12.2024 according to Rule 91] Figure 3-2 is a cycle performance diagram of a button cell assembled with the sodium iron pyrophosphate composite material prepared in Example 1.

[0059] FIG4 is an XRD pattern of the sodium iron pyrophosphate composite material prepared in Example 2.

[0060] FIG5 is an SEM image of the sodium iron pyrophosphate composite material prepared in Example 2.

[0061] [Corrected 13.12.2024 according to Rule 91] Figure 6-1 shows the charge and discharge curves of a button cell assembled with the sodium iron pyrophosphate composite material prepared in Example 2; [0061.1] [Corrected 13.12.2024 according to Rule 91] Figure 6-2 is a cycle performance diagram of a button cell assembled with the sodium iron pyrophosphate composite material prepared in Example 2.

[0062] FIG7 is an XRD pattern of the sodium iron pyrophosphate composite material prepared in Example 3.

[0063] FIG8 is an SEM image of the sodium iron pyrophosphate composite material prepared in Example 3.

[0064] [Corrected 13.12.2024 according to Rule 91] Figure 9-1 shows the charge and discharge curves of the button battery assembled with the sodium iron pyrophosphate composite material prepared in Example 3. [0064.1] [Corrected 13.12.2024 according to Rule 91] Figure 9-2 is a cycle performance diagram of a button cell assembled with the sodium iron pyrophosphate composite material prepared in Example 3.

[0065] FIG10 is an XRD pattern of the sodium iron pyrophosphate composite material prepared in Example 4.

[0066] FIG11 is an SEM image of the sodium iron pyrophosphate composite material prepared in Example 4.

[0067] [Corrected 13.12.2024 according to Rule 91] Figure 12-1 shows the charge and discharge curves of the button battery assembled with the sodium iron pyrophosphate composite material prepared in Example 4. [0067.1] [Corrected 13.12.2024 according to Rule 91] Figure 12-2 is a cycle performance diagram of a button cell assembled with the sodium iron pyrophosphate composite material prepared in Example 4.

[0068] FIG13 is an XRD pattern of the sodium iron pyrophosphate composite material prepared in Comparative Example 1.

[0069] FIG14 is an SEM image of the sodium iron pyrophosphate composite material prepared in Comparative Example 1.

[0070] [Corrected 13.12.2024 according to Rule 91] Figure 15-1 shows the charge and discharge curves of the button battery assembled with the sodium iron pyrophosphate composite material prepared in Comparative Example 1. [0070.1] [Corrected 13.12.2024 according to Rule 91] Figure 15-2 is a cycle performance diagram of the button battery assembled with the sodium iron pyrophosphate composite material prepared in Comparative Example 1.

[0071] FIG16 is an XRD pattern of the sodium iron pyrophosphate composite material prepared in Comparative Example 2.

[0072] FIG17 is an SEM image of the sodium iron pyrophosphate composite material prepared in Comparative Example 2.

[0073] [Corrected 13.12.2024 according to Rule 91] Figure 18-1 shows the charge and discharge curves of the button battery assembled with the sodium iron pyrophosphate composite material prepared in Comparative Example 2. [0073.1] [Corrected 13.12.2024 according to Rule 91] Figure 18-2 is a cycle performance diagram of the button battery assembled with the sodium iron pyrophosphate composite material prepared in Comparative Example 2. DETAILED DESCRIPTION

[0074] To facilitate understanding of the present application, the following will provide a more comprehensive and detailed description of the present application in conjunction with the accompanying drawings and preferred embodiments of the specification, but the scope of protection of the present application is not limited to the following specific embodiments.

[0075] Unless otherwise defined, all technical terms used hereinafter have the same meanings as those generally understood by those skilled in the art. The technical terms used herein are only for the purpose of describing specific embodiments and are not intended to limit the scope of protection of this application.

[0076] Unless otherwise specified, all raw materials, reagents, instruments and equipment used in this application can be purchased from the market or prepared by existing methods.

[0077] Example 1

[0078] (1) NH4FePO4·H2O, Na2CO3, Na4P2O7 and glucose (5% of the total mass of the above substances) were weighed in a molar ratio of 3:0.96:0.54.

[0079] (2) Place the above substances in a sand mill, add sand milling media, and wet sand mill for 4 hours.

[0080] (3) After drying the sand-milled slurry, the temperature was raised at a rate of 10°C / min in an inert gas atmosphere and sintered at 500°C for 8 h to obtain the sodium iron pyrophosphate composite material Na4Fe 2.94(PO4)2P2O7@C.

[0081] After testing, the XRD pattern after deducting the baseline is shown in Figure 1, and the SEM image is shown in Figure 2. The size of the primary particles of the sodium iron pyrophosphate composite material is mainly distributed in the range of 100 to 200 nm, and the secondary particles are formed by the agglomeration of the primary particles, and the size is mainly distributed in the range of 300 to 800 nm. The carbon content is measured by a carbon-sulfur analyzer to be 2.5 wt%. The strongest characteristic diffraction peak peak intensity I of the diffraction peak at 2θ = 33.6° ± 0.2° in the XRD diffraction pattern 1 obtained under CuKα radiation is T , and the characteristic diffraction peak intensity I of NaFePO4(Marticite) at 2θ=32.8°±0.2° O , after deducting the baseline, the peak intensity ratio I T / I O Evaluate the relative content of NaFePO4 (Marticite) in the material. T / I O Defined as Ir, the Ir of the material in this embodiment is 6.19.

[0082] [Corrected 13.12.2024 according to Rule 91] (4) The sodium iron pyrophosphate composite material Na4Fe 2.94 (PO4)2P2O7@C and sodium metal sheet were assembled into a button battery for testing. The assembly of button battery: positive electrode material Na4Fe 2.94 (PO4)2P2O7@C is coated on aluminum foil. The negative electrode material is metallic sodium. The separator used is Whatman GF / B. The electrolyte formula is 1M NaClO4 dissolved in V%EC:PC=1:1. The battery charge and discharge voltage range is 1.7~3.7V. The charge and discharge curves and cycle performance diagrams are shown in Figures 3-1 and 3-2. Its electrochemical 0.1C discharge capacity is 102.3mAh / g, and the capacity retention rate after 100 cycles at 1C is 96.8%.

[0083] Example 2

[0084] (1) NH4FePO4·H2O, Na2CO3, Na4P2O7 and glucose (10% of the total mass of the above substances) were weighed according to the stoichiometric ratio of 3:0.98:0.52.

[0085] (2) Place the above substances in a sand mill, add sand milling media, and wet sand mill for 5 hours.

[0086] (3) After the sand-milled slurry was dried, the temperature was raised at a rate of 13°C / min in an inert gas atmosphere and sintered at 530°C for 10 h to obtain the sodium iron pyrophosphate composite material Na4Fe 2.97(PO4)2P2O7@C. The baseline-subtracted XRD pattern is shown in Figure 4, and the SEM image is shown in Figure 5. The primary particles of the sodium iron pyrophosphate composite material are primarily distributed in the range of 100 to 200 nm, while the secondary particles, formed by agglomeration of the primary particles, are primarily distributed in the range of 300 to 800 nm. The carbon content, as measured by a carbon-sulfur analyzer, is 1.8 wt%. The Ir content of the material of this embodiment is 12.6.

[0087] [Corrected 13.12.2024 according to Rule 91] (4) The sodium iron pyrophosphate composite material Na4Fe 2.97 (PO4)2P2O7@C and sodium metal sheet were assembled into a button battery for testing. The assembly of button battery: positive electrode material Na4Fe 2.97 (PO4)2P2O7@C is coated on aluminum foil. The negative electrode material is metallic sodium. The separator used is Whatman GF / B. The electrolyte formula is 1M NaClO4 dissolved in V%EC:PC=1:1. The battery charge and discharge voltage range is 1.7~3.7V. The charge and discharge curves and cycle performance diagrams are shown in Figures 6-1 and 6-2. The 0.1C discharge capacity is 109.1mAh / g, and the capacity retention rate after 100 cycles at 1C is 95.8%.

[0088] Example 3

[0089] (1) NH4FePO4·H2O, Na2CO3, Na4P2O7 and glucose (10% of the total mass of the above substances) were weighed according to the stoichiometric ratio of 3:0.98:0.52.

[0090] (2) Place the above substances in a sand mill, add sand milling media, and wet sand mill for 5 hours.

[0091] (3) After the sand-milled slurry was dried, the temperature was raised at a rate of 13°C / min in an inert gas atmosphere and sintered at 530°C for 12 h to obtain the sodium iron pyrophosphate composite material Na4Fe 2.97 (PO4)2P2O7@C. The baseline-subtracted XRD pattern is shown in Figure 7, and the SEM image is shown in Figure 8. The primary particles of the sodium ferric pyrophosphate composite material are primarily distributed in the range of 100 to 200 nm, while the secondary particles, formed by agglomeration of the primary particles, are primarily distributed in the range of 300 to 800 nm. The carbon content, measured using a carbon-sulfur analyzer, is 2.8 wt%. The Ir content of the material in this example is 13.

[0092] (4) The sodium iron pyrophosphate composite material Na4Fe 2.97 (PO4)2P2O7@C and sodium metal sheet were assembled into a button battery for testing. The assembly of button battery: positive electrode material Na4Fe 2.97(PO4)2P2O7@C is coated on aluminum foil. The negative electrode material is metallic sodium, the separator used is Whatman GF / B, and the electrolyte formula is 1M NaClO4 dissolved in V%EC:PC=1:1. The battery charge and discharge voltage range is 1.7~3.7V. The charge and discharge curves and cycle performance diagram are shown in Figure 9. The 0.1C discharge capacity is 113.6mAh / g, and the capacity retention rate after 100 cycles at 1C is 96.4%.

[0093] Example 4

[0094] (1) NH4FePO4·H2O, Na2CO3, Na4P2O7 and glucose (20% of the total mass of the above substances) were weighed according to the stoichiometric ratio of 3:1:0.5.

[0095] (2) Place the above substances in a sand mill, add sand milling media, and wet sand mill for 6 hours.

[0096] (3) After drying the sand-milled slurry, the temperature was raised at a rate of 15°C / min in an inert gas atmosphere and sintered at 550°C for 12 h to obtain a sodium ferric pyrophosphate composite material Na4Fe3(PO4)2P2O7@C. The XRD pattern after baseline subtraction is shown in FIG10 , and the SEM image is shown in FIG11 . The size of the primary particles of the sodium ferric pyrophosphate composite material is mainly distributed in the range of 100 to 200 nm, and the secondary particles are formed by agglomeration of the primary particles and are mainly distributed in the range of 300 to 800 nm. The carbon content was measured by a carbon-sulfur analyzer to be 3 wt %. The Ir of the material of this embodiment is 10.7.

[0097] [Corrected 13.12.2024 according to Rule 91](4) The sodium iron pyrophosphate composite material Na4Fe3(PO4)2P2O7@C prepared by the above method was assembled with a sodium metal sheet into a button cell for testing. The button cell assembly is as follows: the positive electrode material Na4Fe3(PO4)2P2O7@C is coated on an aluminum foil, the negative electrode material is sodium metal, the separator used is Whatman GF / B, the electrolyte formula is 1M NaClO4 dissolved in V%EC:PC=1:1, the battery charge and discharge voltage range is 1.7~3.7V, the charge and discharge curves and cycle performance diagrams are shown in Figures 12-1 and 12-2, the 0.1C discharge capacity is 103.3mAh / g, and the capacity retention rate after 100 cycles at 1C is 98.03%.

[0098] Comparative Example 1

[0099] (1) NH4FePO4·H2O, Na2CO3, Na4P2O7 and glucose (10% of the total mass of the above substances) were weighed according to the stoichiometric ratio of 3:0.98:0.52.

[0100] (2) Place the above substances in a sand mill, add sand milling media, and wet sand mill for 4 hours.

[0101] (3) After drying the sand-milled slurry, the temperature was raised in an inert gas atmosphere at a rate of 13°C / min in the temperature ranges of 25-300°C and 400-530°C, and at a rate of 2°C / min in the temperature range of 300-400°C, and sintered at 530°C for 12 h to obtain the composite material Na4Fe 3(1-x) (PO4)2P2O7@C, 0≤x≤0.02. The XRD pattern after baseline subtraction is shown in Figure 13, and the SEM image is shown in Figure 14. The Ir value of the material is 5.6.

[0102] In order to control the formation of the impurity phase, although the same raw material ratios as in Examples 2 and 3 were used in this comparative example, the final product did not have a clear chemical formula, so the above chemical formula containing x was used to represent it.

[0103] [Corrected 13.12.2024 according to Rule 91] (4) The material Na4Fe prepared by the above method 3(1-x) (PO4)2P2O7@C and sodium metal sheet were assembled into a button battery for testing. The assembly of button battery: positive electrode material Na4Fe 3(1-x) (PO4)2P2O7@C is coated on aluminum foil. The negative electrode material is metallic sodium. The separator used is Whatman GF / B. The electrolyte formula is 1M NaClO4 dissolved in V%EC:PC=1:1. The battery charge and discharge voltage range is 1.7~3.7V. The charge and discharge curves and cycle performance diagrams are shown in Figures 15-1 and 15-2. The 0.1C discharge capacity is 93.9mAh / g, and the capacity retention rate after 100 cycles at 1C is 96.9%.

[0104] Comparative Example 2

[0105] (1) NH4FePO4·H2O, Na2CO3, Na4P2O7 and glucose (10% of the total mass of the above substances) were weighed according to the stoichiometric ratio of 3:0.94:0.56.

[0106] (2) Place the above substances in a sand mill, add sand milling media, and wet sand mill for 4 hours.

[0107] (3) After the sand-milled slurry was dried, the temperature was raised at a rate of 15°C / min in an inert gas atmosphere and sintered at 530°C for 12 h to obtain the composite material Na4Fe 2.91 (PO4)2P2O7@C. After testing, its XRD pattern after deducting the baseline is shown in Figure 16, and the SEM image is shown in Figure 17. The Ir of the material is 14.2.

[0108] [Corrected 13.12.2024 according to Rule 91] (4) The material Na4Fe prepared by the above method 2.91 (PO4)2P2O7@C and sodium metal sheet were assembled into a button battery for testing. The assembly of button battery: positive electrode material Na4Fe 2.91 (PO4)2P2O7@C is coated on aluminum foil. The negative electrode material is metallic sodium. The separator used is Whatman GF / B. The electrolyte formula is 1M NaClO4 dissolved in V%EC:PC=1:1. The battery charge and discharge voltage range is 1.7~3.7V. The charge and discharge curves and cycle performance diagrams are shown in Figures 18-1 and 18-2. The 0.1C discharge capacity is 98.4mAh / g, and the capacity retention rate after 100 cycles at 1C is 93.7%.

[0109] Table 1

[0110] The electrochemical properties and Ir values ​​of each embodiment and comparative example are shown in Table 1. It can be seen from the table that when 6.19≤Ir≤13, that is, when the spinel-type NaFePO4 phase content in the obtained sodium iron pyrophosphate composite material is within a certain content range, the corresponding electrochemical properties such as battery capacity and cycle performance are good. Based on this discovery, the process parameters of the method for preparing composite sodium iron phosphate by the solid phase method in this application can be evaluated and optimized, and the method for preparing composite sodium iron phosphate can be optimized by optimizing the ratio of raw materials in step (1), the heating rate and sintering temperature and holding time in step (2).

[0111] Optimization of process parameters in Comparative Example 1:

[0112] Example 2 is equivalent to the optimized condition result of Comparative Example 1. Compared with Example 2, Comparative Example 1 extends the NaFePO4 phase formation temperature zone time during heating, and the final Ir is too low and the performance is poor, which shows the feasibility and rationality of controlling the heating rate condition for reducing the NaFePO4 phase.

[0113] Optimization of process parameters in Comparative Example 2:

[0114] The optimized parameter results of Examples 1 and 2 are equivalent to those of Comparative Example 2. Comparative Example 2 shows that the NaFePO4 phase is also one of the structural phases of composite sodium iron phosphate. Too low a content will also destroy the structural stability of NFPP itself and reduce the material performance. In this Comparative Example 2, the Fe source ratio is further reduced (by 3%). Ultimately, Ir is too high and the performance is poor. The decline in cycle performance indirectly illustrates that the structural stability of the material itself is reduced in this case.

[0115] The above is only a preferred embodiment of the present application. It should be pointed out that for ordinary technicians in this technical field, several improvements and modifications can be made without departing from the principles of the present application. These improvements and modifications should also be regarded as the scope of protection of the present application.

Claims

1. A sodium iron pyrophosphate composite material, wherein: The sodium iron pyrophosphate composite material has a spinel NaFePO4 phase; the sodium iron pyrophosphate composite material is subjected to XRD diffraction under CuKα radiation, and the obtained XRD diffraction spectrum has a characteristic diffraction peak T of sodium iron pyrophosphate and a characteristic diffraction peak O of spinel NaFePO4 at 2θ of 33.6±0.2° and 32.8±0.2°, and the peak intensities are respectively I T and I O , let the diffraction peak intensity ratio Ir = I T / I O , where 6.19≤Ir≤13.

2. The sodium iron pyrophosphate composite material according to claim 1, wherein It includes sodium ferric pyrophosphate and a carbon layer coated on the surface of the sodium ferric pyrophosphate, and the chemical formula is Na4Fe 3(1-x) (PO4)2P2O7@C, where 0≤x≤0.

02.

3. The sodium iron pyrophosphate composite material according to claim 1 or 2, wherein The carbon content of the sodium iron pyrophosphate composite material is 2-5wt%.

4. A preparation process of a sodium iron pyrophosphate composite material, wherein: The following steps are involved: (1) Weigh the synthetic Na4Fe according to the preset feeding ratio 3(1-x) The raw materials of (PO4)2P2O7 and the carbon source are wet-grinded and mixed to obtain a mixture; (2) After the mixture is dried, it is sintered in a protective atmosphere according to preset sintering parameters to obtain a sodium iron pyrophosphate composite material Na4Fe 3(1-x) (PO4)2P2O7@C; Among them, 0≤x≤0.

02.

5. The preparation process according to claim 4, wherein: In step (1): The synthesis of Na4Fe 3(1-x) The raw materials of (PO4)2P2O7 include an iron source and a sodium source; the iron source is ammonium ferrous phosphate, and the sodium source includes sodium pyrophosphate and a sodium source that does not contain P.

6. The preparation process according to claim 5, wherein: The P-free sodium source is selected from one or more of sodium carbonate, ammonium bicarbonate, sodium acetate and sodium citrate.

7. The preparation process according to claim 6, wherein: The molar ratio of the ammonium ferrous phosphate, the sodium source not containing P and the sodium pyrophosphate is 3: (0.96-1): (0.5-0.54).

8. The preparation process according to claim 4, wherein: In step (1): The carbon source is one or more of glucose, sucrose, starch, and polyethylene glycol; the amount of the carbon source is the amount of synthesized Na4Fe 3(1-x) (PO4)2P2O7 accounts for 5 to 20% of the mass of the raw material.

9. The preparation process according to claim 4, wherein: In step (2), the preset sintering parameters include the sintering heating rate, the sintering temperature and the sintering time, wherein at least one of the following conditions is satisfied: The heating rate of the sintering is 10-15°C / min; The sintering temperature is 500-550°C; The sintering time is 8 to 12 hours.

10. A method for evaluating a sodium iron pyrophosphate composite material, wherein: The following steps are involved: S1. Performing an XRD diffraction test on the sodium iron pyrophosphate composite material under CuKα radiation, and obtaining an XRD diffraction pattern, and calculating the corresponding performance test value according to the peak intensity of the required characteristic diffraction peak; S2. Obtain performance evaluation results by comparing performance test values ​​with performance reference values.

11. The evaluation method according to claim 10, wherein: In step S1, the performance test value is Ir', Ir'=I T ' / I O ', where I T ' is the peak intensity of the characteristic diffraction peak T at 2θ of 33.6±0.2° in the XRD diffraction pattern, I O ' is the peak intensity of the characteristic diffraction peak O of the XRD diffraction spectrum at 2θ of 32.8±0.2°; In step S2, the performance reference value is Ir, and its interval range is 6.19≤Ir≤13; when Ir' is within the interval range of Ir, the performance evaluation result of the material is excellent.

12. A method for optimizing the preparation process of sodium iron pyrophosphate composite material, wherein: The sodium iron pyrophosphate composite material prepared by the preparation process of any one of claims 4 to 9 or the sodium iron pyrophosphate composite material of any one of claims 1 to 3 is evaluated using the evaluation method of claim 10 or 11, further comprising the following steps: S3. By comparing the performance test value and the performance reference value, it is determined whether the performance test value is within the range of the performance reference value; If the performance test value is not within the performance reference value range, repeat steps (1) and ( 2), step S1, step S2, and optimize and adjust the process parameters in step (1) or (2) according to the comparison results until the performance test value is within the interval range of the performance reference value, and then output the optimized process parameters.

13. The optimization method according to claim 12, wherein: The process parameters in step (1) or (2) are optimized and adjusted according to the comparison results, wherein the process parameters include the amount of iron source added to the preset feed in step (1) and / or the preset sintering parameters in step (2).

14. The optimization method according to claim 12 or 13, wherein: When Ir' is less than the lower limit of the interval of Ir, reduce the amount of iron source added and / or increase the heating rate; when Ir' is greater than the upper limit of the interval of Ir, increase the amount of iron source added and / or reduce the heating rate.

15. Use of the sodium iron pyrophosphate composite material according to any one of claims 1 to 3, or the sodium iron pyrophosphate composite material prepared by the preparation process according to any one of claims 4 to 9, or the sodium iron pyrophosphate composite material evaluated as excellent by the evaluation method according to claim 10 or 11, or the sodium iron pyrophosphate composite material prepared by the sodium iron pyrophosphate composite material preparation process optimized by the optimization method according to any one of claims 12 to 14, wherein: Application as a positive electrode material in sodium ion batteries.

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