WO2025113137A1 - Sodium ferric phosphate pyrophosphate composite material, and preparation process, evaluation method and process optimization method therefor and use thereof - Google Patents

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 I_T and I_O, and the diffraction peak intensity ratio Ir is equal to I_T/I_O, wherein 6.19≤Ir≤13.

Description

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

This application claims the priority of the Chinese patent application filed with the China Patent Office on November 29, 2023, with application number 202311612427.2 and application name “Sodium iron pyrophosphate composite material and its preparation process, evaluation method, process optimization method and application”, all contents of which are incorporated by reference in this application.

Technical Field

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

Sodium-ion batteries are rocking-chair batteries, which are secondary batteries that rely on the insertion and extraction of ions between the positive and negative electrodes. Both the positive and negative electrode materials allow sodium ions to be reversibly inserted and extracted. Research on sodium-ion batteries has been going on since 1982, and has made great progress in recent years and has met the conditions for commercial application. At present, the positive electrode materials of sodium-ion batteries mainly include oxides, Prussian blue and polyanions.

As a polyanion sodium ferric pyrophosphate 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.

At present, the common synthesis method of this material is the liquid phase method, which has high requirements on reaction process conditions, complex production equipment, and great difficulty in mass production, and is not suitable for large-scale industrial applications. In addition, there is also a scheme for preparing sodium iron pyrophosphate material by wet homogeneous sand milling combined with spray drying. However, in order to ensure the acquisition of pure phase Na ₄ Fe ₃ (PO ₄ ) ₂ (P ₂ O ₇ ), 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 operation process and reaction equipment. In addition, the capacity performance of sodium iron pyrophosphate composite materials obtained by the existing synthesis process is limited, and there is still room for further improvement.

Summary of the invention

In view of the above technical problems, one of the objectives of the present application is to provide a sodium iron pyrophosphate composite material.

The second purpose of the present application is to provide a preparation process of sodium iron pyrophosphate composite material.

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

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

The fifth object of the present application is to provide a sodium iron pyrophosphate composite material for use as a positive electrode material for sodium ion batteries in sodium ion batteries.

Compared with the prior art that pursues to obtain pure phase Na ₄ Fe ₃ (PO ₄ ) ₂ (P ₂ O ₇ ), the present application creatively discovers that when the sodium iron pyrophosphate composite material has a specific content of spinel NaFePO ₄ phase, the capacity performance of the sodium iron pyrophosphate composite material can be effectively improved, but the NaFePO ₄ phase in the sodium iron pyrophosphate composite material may also play a certain role as a structural phase. 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.

Therefore, 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 in the obtained XRD diffraction pattern, the sodium iron pyrophosphate composite material 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 _IT and _IO respectively, and the diffraction peak intensity ratio Ir= _IT / _IO , wherein 6.19≤Ir≤13.

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 has a chemical formula of Na ₄ Fe _3(1-x) (PO ₄ ) ₂ P ₂ O ₇ @C, wherein 0≤x≤0.02.

Further preferably, the carbon content of the sodium iron pyrophosphate composite material is 2-5 wt %.

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 of the secondary particles is mainly distributed in the range of 300 to 800 nm.

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.

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

(1) weighing raw materials for synthesizing Na ₄ Fe _3(1-x) (PO ₄ ) ₂ P ₂ O ₇ and an appropriate amount of carbon source according to a preset feeding ratio, performing wet grinding and mixing to obtain a mixed material;

(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 Na ₄ Fe _3(1-x) (PO ₄ ) ₂ P ₂ O ₇ @C;

Among them, 0≤x≤0.02.

Furthermore, in step (1): the raw materials for synthesizing Na ₄ Fe _3(1-x) (PO ₄ ) ₂ P ₂ O ₇ include an iron source and a sodium source.

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

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

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).

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

More preferably, the amount of the carbon source is 5-20% of the mass of the raw material for synthesizing Na ₄ Fe _3(1-x) (PO ₄ ) ₂ P ₂ O ₇ .

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.

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

Furthermore, 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.

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.

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

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.

Further,

The performance test value in step S1 is Ir', Ir'= _IT '/ _I0 ', wherein _IT ' is the peak intensity of the characteristic diffraction peak T of the XRD diffraction spectrum at 2θ of 33.6±0.2°, and _I0 ' is the peak intensity of the characteristic diffraction peak O of the XRD diffraction spectrum at 2θ of 32.8±0.2°;

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.

The present application study found that there are two possible reaction pathways in the synthesis reaction of sodium iron pyrophosphate composite material, namely: 4NH ₄ FePO ₄ ·H ₂ O+Na ₂ CO ₃ +Na ₄ P ₂ O ₇ →2NaFePO ₄ +2Na ₂ FeP ₂ O ₇ +CO ₂ +4NH ₃ +6H ₂ O (1)6NH ₄ FePO ₄ ·H ₂ O+2Na ₂ CO ₃ +Na ₄ P ₂ O ₇ →2Na ₄ Fe ₃ (PO ₄ ) ₂ (P ₂ O ₇ )+6NH ₃ +9H ₂ O+2CO ₂ (2)

During the reaction, 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 NaFePO ₄ is approximately 300-400°C, the electrochemically inert NaFePO ₄ has a stronger tendency to form the target product than reaction (2).

Based on the above findings, the present application can reduce the _phase formation time of NaFePO4 by increasing the heating rate, and combine 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.

In this regard, the present application discloses a method for optimizing the preparation process of a sodium iron pyrophosphate composite material, using 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 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 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.

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).

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.

The present application also discloses the use of the sodium iron pyrophosphate composite material described above, 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 for sodium ion batteries in sodium ion batteries.

This application has the following beneficial effects:

(1) In the prior art, pure phase Na ₄ Fe ₃ (PO ₄ ) ₂ (P ₂ O ₇ ) is generally attempted to be obtained by process improvement, and the electrochemically inert spinel-type NaFePO ₄ phase is reduced to avoid its influence on the electrochemical performance of the material. The present application creatively finds that when the sodium iron pyrophosphate composite material has a specific content of spinel-type NaFePO ₄ phase, the capacity performance of the material can be effectively improved. The test shows that the sodium iron pyrophosphate composite material with a specific content of NaFePO ₄ has an average capacity of about 10% higher than that of materials with a lower or higher content.

(2) The present application limits the content of the spinel _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.

(3) The present application proposes a solid phase synthesis method of composite sodium iron phosphate based on NH ₄ FePO ₄ ·H ₂ O as an iron source. The raw material cost is low, the formula is easy to control, the synthesized product NaFePO ₄ (Marticite) content is appropriate, the process is simple, the economic benefit is high, and the prepared material has excellent electrochemical properties.

(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

In order to more clearly illustrate the embodiments of the present application or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. 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 paying any creative work.

FIG. 1 is an XRD diagram of the sodium iron pyrophosphate composite material prepared in Example 1.

FIG. 2 is a SEM image of the sodium iron pyrophosphate composite material prepared in Example 1.

[Corrected 13.12.2024 in accordance with Article 91]
FIG3-1 is a charge and discharge curve of a button cell assembled with the sodium iron pyrophosphate composite material prepared in Example 1. ...

[Corrected 13.12.2024 in accordance with Article 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.

FIG. 4 is an XRD diagram of the sodium iron pyrophosphate composite material prepared in Example 2.

FIG. 5 is a SEM image of the sodium iron pyrophosphate composite material prepared in Example 2.

[Corrected 13.12.2024 in accordance with Article 91]
FIG6-1 is a charge and discharge curve of a button cell assembled with the sodium iron pyrophosphate composite material prepared in Example 2;

[Corrected 13.12.2024 in accordance with Article 91]
Figure 6-2 is a cycle performance diagram of the button battery assembled with the sodium iron pyrophosphate composite material prepared in Example 2.

FIG. 7 is an XRD diagram of the sodium iron pyrophosphate composite material prepared in Example 3.

FIG8 is a SEM image of the sodium iron pyrophosphate composite material prepared in Example 3.

[Corrected 13.12.2024 in accordance with Article 91]
FIG9-1 is a charge and discharge curve of a button cell assembled with the sodium iron pyrophosphate composite material prepared in Example 3.

[Corrected 13.12.2024 in accordance with Article 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.

FIG10 is an XRD diagram of the sodium iron pyrophosphate composite material prepared in Example 4.

FIG. 11 is a SEM image of the sodium iron pyrophosphate composite material prepared in Example 4.

[Corrected 13.12.2024 in accordance with Article 91]
FIG12-1 is a charge and discharge curve of a button cell assembled with the sodium iron pyrophosphate composite material prepared in Example 4.

[Corrected 13.12.2024 in accordance with Article 91]
FIG12-2 is a cycle performance diagram of a button cell assembled with the sodium iron pyrophosphate composite material prepared in Example 4.

FIG13 is an XRD diagram of the sodium iron pyrophosphate composite material prepared in Comparative Example 1.

FIG14 is a SEM image of the sodium iron pyrophosphate composite material prepared in Comparative Example 1.

[Corrected 13.12.2024 in accordance with Article 91]
Figure 15-1 is the charge and discharge curve of the button battery assembled with the sodium iron pyrophosphate composite material prepared in Comparative Example 1.

[Corrected 13.12.2024 in accordance with Article 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.

Figure 16 is an XRD diagram of the sodium iron pyrophosphate composite material prepared in Comparative Example 2.

Figure 17 is a SEM image of the sodium iron pyrophosphate composite material prepared in Comparative Example 2.

[Corrected 13.12.2024 in accordance with Article 91]
Figure 18-1 is the charge and discharge curve of the button battery assembled with the sodium iron pyrophosphate composite material prepared in Comparative Example 2.

[Corrected 13.12.2024 in accordance with Article 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

In order to facilitate the 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 protection scope of the present application is not limited to the following specific embodiments.

Unless otherwise defined, all professional terms used below have the same meanings as those generally understood by those skilled in the art. The professional 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.

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

Example 1

(1) NH ₄ FePO ₄ ·H ₂ O, Na ₂ CO ₃ , Na ₄ P ₂ O ₇ and 5% of the total mass of glucose were weighed respectively according to the molar ratio of 3:0.96:0.54.

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

(3) After the sand-milled slurry was dried, the temperature was increased at a rate of 10°C/min in an inert gas atmosphere and sintered at 500°C for 8h to obtain a sodium iron pyrophosphate composite material Na ₄ Fe _2.94 (PO ₄ ) ₂ P ₂ O ₇ @C.

After detection, the XRD pattern after deducting the baseline is shown in FIG1 , and the SEM pattern is shown in FIG2 . The size of the primary particles of the sodium iron pyrophosphate composite material is mainly distributed in 100-200 nm, and the secondary particles are formed by agglomeration of the primary particles, and the size is mainly distributed in 300-800 nm. The carbon content measured by the carbon-sulfur analyzer is 2.5wt%. The relative content of NaFePO _{4 (Marticite) in the material is evaluated by the peak intensity ratio IT/IO after deducting the baseline, which is the strongest characteristic diffraction peak peak intensity IT of the diffraction peak at 2θ=33.6°±0.2° in the XRD diffraction pattern 1 obtained under CuKα radiation, and the characteristic diffraction peak intensity IO of NaFePO 4 ( Marticite ) at 2θ} =32.8°±0.2°. This peak intensity ratio _IT / _IO is defined as Ir, and the Ir of the material in this embodiment is 6.19.

[Corrected 13.12.2024 in accordance with Article 91]
(4) The sodium iron pyrophosphate composite material Na ₄ Fe _2.94 (PO ₄ ) ₂ P ₂ O ₇ @C prepared by the above method was assembled with a metal sodium sheet into a button cell for testing. The assembly of the button cell is as follows: the positive electrode material Na ₄ Fe _2.94 (PO ₄ ) ₂ P ₂ O ₇ @C is coated on an aluminum foil, the negative electrode material is metal sodium, the separator used is Whatman GF/B, the electrolyte formula is 1M NaClO ₄ 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%.

Example 2

(1) NH ₄ FePO ₄ ·H ₂ O, Na ₂ CO ₃ , Na ₄ P ₂ O ₇ and 10% of the total weight of glucose were weighed respectively according to the stoichiometric ratio of 3:0.98:0.52.

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

(3) After drying the sand-milled slurry, 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 a sodium pyrophosphate iron phosphate composite material Na ₄ Fe _2.97 (PO ₄ ) ₂ P ₂ O ₇ @C. After testing, the XRD pattern after deducting the baseline is shown in FIG4 , and the SEM pattern is shown in FIG5 . The size of the primary particles of the sodium pyrophosphate iron phosphate composite material is mainly distributed in 100-200 nm, and the secondary particles are formed by agglomeration of the primary particles, and the size is mainly distributed in 300-800 nm. The carbon content measured by a carbon-sulfur analyzer is 1.8wt%, and the Ir of the material of this embodiment is 12.6.

[Corrected 13.12.2024 in accordance with Article 91]
(4) The sodium iron pyrophosphate composite material Na ₄ Fe _2.97 (PO ₄ ) ₂ P ₂ O ₇ @C prepared by the above method was assembled with a metal sodium sheet into a button cell for testing. The assembly of the button cell is as follows: the positive electrode material Na ₄ Fe _2.97 (PO ₄ ) ₂ P ₂ O ₇ @C is coated on an aluminum foil, the negative electrode material is metal sodium, the separator used is Whatman GF/B, the electrolyte formula is 1M NaClO ₄ 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%.

Example 3

(1) NH ₄ FePO ₄ ·H ₂ O, Na ₂ CO ₃ , Na ₄ P ₂ O ₇ and 10% of the total weight of glucose were weighed respectively according to the stoichiometric ratio of 3:0.98:0.52.

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

(3) After drying the sand-milled slurry, the temperature was raised at a rate of 13°C/min in an inert gas atmosphere and sintered at 530°C for 12h to obtain a sodium pyrophosphate iron phosphate composite material Na ₄ Fe _2.97 (PO ₄ ) ₂ P ₂ O ₇ @C. After testing, the XRD pattern after deducting the baseline is shown in FIG7 , and the SEM pattern is shown in FIG8 . The size of the primary particles of the sodium pyrophosphate iron phosphate composite material is mainly distributed in 100-200nm, and the secondary particles are formed by agglomeration of the primary particles, and the size is mainly distributed in 300-800nm. The carbon content measured by a carbon-sulfur analyzer is 2.8wt%. The Ir of the material in this embodiment is 13.

(4) The sodium iron pyrophosphate composite material Na ₄ Fe _2.97 (PO ₄ ) ₂ P ₂ O ₇ @C prepared by the above method was assembled with a metal sodium sheet into a button cell for testing. The assembly of the button cell is as follows: the positive electrode material Na ₄ Fe _2.97 (PO ₄ ) ₂ P ₂ O ₇ @C is coated on an aluminum foil, the negative electrode material is metal sodium, the separator used is Whatman GF/B, the electrolyte formula is 1M NaClO ₄ 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 the 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%.

Example 4

(1) NH ₄ FePO ₄ ·H ₂ O, Na ₂ CO ₃ , Na ₄ P ₂ O ₇ and 20% of the total weight of glucose were weighed according to the stoichiometric ratio of 3:1:0.5.

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

(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 12h to obtain a sodium pyrophosphate iron phosphate composite material Na ₄ Fe ₃ (PO ₄ ) ₂ P ₂ O ₇ @C. After testing, the XRD pattern after deducting the baseline is shown in FIG10 , and the SEM pattern is shown in FIG11 . The size of the primary particles of the sodium pyrophosphate iron phosphate composite material is mainly distributed in 100-200nm, and the secondary particles are formed by agglomeration of the primary particles, and the size is mainly distributed in 300-800nm. The carbon content measured by a carbon-sulfur analyzer is 3wt%, and the Ir of the material of this embodiment is 10.7.

[Corrected 13.12.2024 in accordance with Article 91]
(4) The sodium iron pyrophosphate composite material Na ₄ Fe ₃ (PO ₄ ) ₂ P ₂ O ₇ @C prepared by the above method was assembled with a metal sodium sheet into a button cell for testing. The assembly of the button cell is as follows: the positive electrode material Na ₄ Fe ₃ (PO ₄ ) ₂ P ₂ O ₇ @C is coated on an aluminum foil, the negative electrode material is metal sodium, the separator used is Whatman GF/B, the electrolyte formula is 1M NaClO ₄ 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%.

Comparative Example 1

(1) NH ₄ FePO ₄ ·H ₂ O, Na ₂ CO ₃ , Na ₄ P ₂ O ₇ and 10% of the total weight of glucose were weighed respectively according to the stoichiometric ratio of 3:0.98:0.52.

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

(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 a composite material Na ₄ Fe _3(1-x) (PO ₄ ) ₂ P ₂ O ₇ @C, 0≤x≤0.02. After testing, the XRD pattern after deducting the baseline is shown in Figure 13, and the SEM image is shown in Figure 14. The Ir of the material is 5.6.

In order to control the generation of the impurity phase, although the same raw material ratio as in Examples 2 and 3 was 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.

[Corrected 13.12.2024 in accordance with Article 91]
(4) The material Na ₄ Fe _3(1-x) (PO ₄ ) ₂ P ₂ O ₇ @C prepared by the above method was assembled with a metal sodium sheet into a button cell for testing. The assembly of the button cell is as follows: the positive electrode material Na ₄ Fe _3(1-x) (PO ₄ ) ₂ P ₂ O ₇ @C is coated on an aluminum foil, the negative electrode material is metal sodium, the separator used is Whatman GF/B, the electrolyte formula is 1M NaClO ₄ 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%.

Comparative Example 2

(1) NH ₄ FePO ₄ ·H ₂ O, Na ₂ CO ₃ , Na ₄ P ₂ O ₇ and 10% of the total weight of glucose were weighed respectively according to the stoichiometric ratio of 3:0.94:0.56.

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

(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 530°C for 12 h to obtain a composite material Na ₄ Fe _2.91 (PO ₄ ) ₂ P ₂ O ₇ @C. After testing, the XRD pattern after deducting the baseline is shown in FIG16 , and the SEM image is shown in FIG17 , and the Ir of the material is 14.2.

[Corrected 13.12.2024 in accordance with Article 91]
(4) The material Na ₄ Fe _2.91 (PO ₄ ) ₂ P ₂ O ₇ @C prepared by the above method was assembled with a metal sodium sheet into a button cell for testing. The assembly of the button cell is as follows: the positive electrode material Na ₄ Fe _2.91 (PO ₄ ) ₂ P ₂ O ₇ @C is coated on an aluminum foil, the negative electrode material is metal sodium, the separator used is Whatman GF/B, the electrolyte formula is 1M NaClO ₄ 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%.

Table 1

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 NaFePO ₄ 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 the holding time in step (2).

Optimization of process parameters in Comparative Example 1:

Example 2 is equivalent to the optimized condition result of Comparative Example 1. Compared with Example 2, Comparative Example 1 prolongs the NaFePO4 _phase formation temperature zone time during heating, and finally Ir is too low and the performance is poor, which shows the feasibility and rationality of controlling the heating rate condition for NaFePO4 _phase reduction.

Optimization of process parameters in Comparative Example 2:

Examples 1/2 are equivalent to the optimization parameter results of Comparative Example 2. Comparative Example 2 shows that _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. This Comparative Example 2 further reduces the Fe source ratio (reduced by 3%), and finally Ir is too high and the performance is poor. The decline in cycle performance indirectly indicates that the structural stability of the material itself is reduced in this case.

The above is only a preferred implementation 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 (15)

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 thereof are _IT and _IO respectively, and the diffraction peak intensity ratio Ir= _IT / _IO , wherein 6.19≤Ir≤13. The sodium iron pyrophosphate composite material according to claim 1, comprising sodium iron pyrophosphate and a carbon layer coated on the surface of the sodium iron pyrophosphate, and the chemical formula is Na ₄ Fe _3(1-x) (PO ₄ ) ₂ P ₂ O ₇ @C, wherein 0≤x≤0.02. 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 to 5 wt%. A preparation process of a sodium iron pyrophosphate composite material, comprising the following steps: (1) weighing raw materials for synthesizing Na ₄ Fe _3(1-x) (PO ₄ ) ₂ P ₂ O ₇ and a carbon source according to a preset feeding ratio, performing wet grinding and mixing to obtain a mixed material; (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 Na ₄ Fe _3(1-x) (PO ₄ ) ₂ P ₂ O ₇ @C; Among them, 0≤x≤0.02. The preparation process according to claim 4, wherein in step (1): The raw materials for synthesizing Na ₄ Fe _3(1-x) (PO ₄ ) ₂ P ₂ O ₇ 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 not containing P. 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. The preparation process according to claim 6, wherein the molar ratio of the ammonium ferrous phosphate, the P-free sodium source and the sodium pyrophosphate is 3: (0.96-1): (0.5-0.54). 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 5-20% of the mass of the raw material for synthesizing Na ₄ Fe _3(1-x) (PO ₄ ) ₂ P ₂ O ₇ . 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. A method for evaluating a sodium iron pyrophosphate composite material, comprising the following steps: 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. The evaluation method according to claim 10, wherein: In step S1, the performance test value is Ir', Ir'= _IT '/ _I0 ', wherein _IT ' is the peak intensity of the characteristic diffraction peak T of the XRD diffraction spectrum at 2θ of 33.6±0.2°, and _I0 ' 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. A method for optimizing the preparation process of a 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 described in claim 10 or 11, and 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. 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). According to the optimization method of claim 12 or 13, when Ir' is less than the lower limit of the interval of Ir, the amount of iron source added is 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 increased and/or the heating rate is reduced. The use of the sodium iron pyrophosphate composite material as described in any one of claims 1 to 3, or 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 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 the use of the sodium iron pyrophosphate composite material as a positive electrode material for sodium ion batteries in sodium ion batteries.