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 phosphate pyrophosphate composites, their preparation process, evaluation methods, process optimization methods, and applications

[0001] This application claims priority to Chinese Patent Application No. 202311612427.2, filed on November 29, 2023, entitled "Sodium Iron Pyrophosphate Composite Material and its Preparation Process, Evaluation Method, Process Optimization Method and Application", the entire contents of which are incorporated herein by reference. Technical Field

[0002] This application belongs to the field of sodium-ion battery materials technology, specifically relating to a sodium iron phosphate pyrophosphate composite material and its preparation process, evaluation method, process optimization method and application. Background Technology

[0003] Sodium-ion batteries are rocking-chair batteries, a type of secondary battery that relies on the reciprocating insertion and extraction of ions between 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, reaching a point where they are ready for commercial application. Currently, the main positive electrode materials for sodium-ion batteries include oxides, Prussian blue compounds, and polyanionic compounds.

[0004] Sodium iron pyrophosphate composite material is a polyanionic sodium-ion battery cathode material. This material has a stable sodium ion diffusion channel and good cycle performance. It also has a moderate working voltage and specific capacity, making it an inexpensive and promising sodium-ion battery cathode material.

[0005] Currently, the most common synthesis method for this material is the liquid-phase method, which requires high reaction conditions, complex production equipment, and is difficult to mass-produce, making it unsuitable for large-scale industrial applications. In addition, there is a scheme to prepare sodium iron phosphate 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 demands on the operation process and reaction equipment. Furthermore, the capacity performance of sodium iron phosphate pyrophosphate composite materials obtained by existing synthesis processes is limited, and there is still room for further improvement.

[0006] Summary of the Invention

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

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

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

[0010] The fourth objective of this application is to provide an optimized method for the preparation process of sodium iron phosphate pyrophosphate composite materials.

[0011] The fifth objective of this application is to provide an application of sodium iron phosphate pyrophosphate composite material as a positive electrode material in sodium-ion batteries.

[0012] Compared to the existing technology that seeks to obtain a pure phase of Na4Fe3(PO4)2(P2O7), this application creatively discovers that when the sodium iron phosphate pyrophosphate composite material has a specific content of spinel-type NaFePO4 phase, the capacity performance of the sodium iron phosphate pyrophosphate composite material can be effectively improved. However, the NaFePO4 phase in the sodium iron phosphate 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 damaged, thereby reducing the overall electrochemical performance; while when its content is too high, it will reduce the overall electrochemical performance.

[0013] Therefore, this application provides a sodium iron phosphate pyrophosphate composite material, which has a spinel-type NaFePO4 phase; the sodium iron phosphate pyrophosphate composite material is subjected to XRD diffraction under CuKα radiation, and the obtained XRD diffraction pattern shows the characteristic diffraction peak T of sodium iron phosphate pyrophosphate and the characteristic diffraction peak O of spinel-type NaFePO4 at 2θ of 33.6±0.2° and 32.8±0.2°, respectively, with peak intensities of I and I, respectively. T and I O Let the diffraction peak intensity ratio Ir = I T / I O , where 6.19≤Ir≤13.

[0014] Furthermore, the sodium iron phosphate pyrophosphate composite material comprises sodium iron phosphate pyrophosphate and a carbon layer coated on the surface of the sodium iron phosphate pyrophosphate, with the chemical formula Na₄Fe₂O₃. 3(1-x) (PO4)2P2O7@C, where 0≤x≤0.02.

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

[0016] More preferably, the primary particles of the sodium iron phosphate pyrophosphate composite material are mainly distributed in the range of 100-200 nm, and the secondary particles are formed by the agglomeration of the primary particles, with a size mainly distributed in the range of 300-800 nm.

[0017] Based on the influence of NaFePO4 phase content on material properties, controlling the NaFePO4 phase is the most critical challenge in the synthesis of this material.

[0018] In this regard, this application also provides a preparation process for the aforementioned sodium iron phosphate pyrophosphate composite material, including the following steps:

[0019] (1) Weigh out the synthetic Na4Fe according to the preset feeding ratio. 3(1-x) (PO4)2P2O7 raw material and appropriate carbon source are wet-milled and mixed to obtain a mixture.

[0020] (2) After drying the mixture, sinter it under a protective atmosphere according to the preset sintering parameters to obtain the sodium iron phosphate pyrophosphate composite material Na4Fe. 3(1-x) (PO4)2P2O7@C;

[0021] Where 0 ≤ x ≤ 0.02.

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

[0023] More preferably, the iron source is ferrous ammonium phosphate, and the sodium source includes sodium pyrophosphate and a sodium source that does not contain phosphorus.

[0024] More 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 ferrous ammonium phosphate, a phosphorus-free sodium source, 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 carbon source used is for synthesizing Na4Fe. 3(1-x) The raw material of (PO4)2P2O7 accounts for 5-20% of the mass.

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

[0029] Furthermore, in step (1), the grinding method is vibratory grinding or rolling grinding. More preferably, it is continuous rolling ball milling.

[0030] Further, 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:

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

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

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

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

[0035] This application discloses an evaluation method for sodium iron phosphate pyrophosphate composite materials, comprising the following steps:

[0036] S1. The sodium iron phosphate pyrophosphate composite material was subjected to XRD diffraction test under CuKα radiation, and the XRD diffraction pattern was obtained. The corresponding performance test value was calculated based on the peak intensity of the required characteristic diffraction peaks.

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

[0038] Furthermore,

[0039] The performance test value mentioned in step S1 is Ir', where Ir' = I T ' / I O ', where I T 'I represents the peak intensity of the characteristic diffraction peak T at 2θ = 33.6 ± 0.2° in the XRD diffraction pattern. O '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 mentioned in step S2 is Ir, and its range is 6.19≤Ir≤13; when Ir' is within the range of Ir, the performance evaluation result of the material is excellent.

[0041] The study in this application found that there are two possible reaction pathways in the synthesis reaction of sodium iron phosphate pyrophosphate composite material: 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 of the above reactions may occur and compete with each other. However, since reaction (1) has a greater entropy increase and a greater thermodynamic advantage, the phase formation temperature range of NaFePO4 is about 300-400℃. Therefore, the electrochemically inert NaFePO4 has a stronger tendency to form the target product compared with reaction (2).

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

[0044] In response, this application discloses an optimization method for the preparation process of sodium iron phosphate pyrophosphate composite material. The method for evaluating sodium iron phosphate pyrophosphate composite material is described above, and the sodium iron phosphate pyrophosphate composite material prepared by the above-described composite sodium iron phosphate cathode material preparation process is evaluated. The method further includes the following steps:

[0045] S3. By comparing the performance test value and the performance reference value, determine 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 based on the comparison results. The process parameters include the amount of iron source added in the preset feeding in step (1) and / or the preset sintering parameters in step (2).

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

[0049] This application also discloses the application of the above-described sodium iron phosphate pyrophosphate composite material, or the sodium iron phosphate pyrophosphate composite material prepared by the above-described preparation process, or the sodium iron phosphate pyrophosphate composite material evaluated as superior by the above-described evaluation method, or the sodium iron phosphate pyrophosphate composite material prepared by the above-described optimization method as a positive electrode material for sodium-ion batteries in sodium-ion batteries.

[0050] This application has the following beneficial effects:

[0051] (1) In the prior art, the aim is generally to obtain pure Na4Fe3(PO4)2(P2O7) through process improvement and reduce the electrochemically inert spinel-type NaFePO4 phase to avoid affecting the electrochemical performance of the material. This application creatively discovers that when the sodium iron phosphate pyrophosphate composite material has a specific content of spinel-type NaFePO4 phase, the capacity performance of the material can be effectively improved. Experiments show that the sodium iron phosphate 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) In this application, the content of spinel-type NaFePO4 phase is limited by the characteristic peaks and the ratio of their peak intensities in the XRD spectrum obtained by XRD diffraction of sodium iron phosphate pyrophosphate composite material under CuKα radiation. This ensures improved capacity performance while 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 iron source. The raw materials used are low cost, the formula is easy to control, the synthesized product has a suitable NaFePO4 (Marticite) content, the process is simple, the economic benefits are high, and the prepared material has excellent electrochemical performance.

[0054] (4) The present application evaluates the performance of the prepared composite sodium iron phosphate based on the evaluation method of sodium iron phosphate pyrophosphate composite material. 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 NaFePO4 phase, and thus control the content of NaFePO4 phase in the product, so as to obtain a process that can prepare products with better performance. Attached Figure Description

[0055] To more clearly illustrate the technical solutions in the embodiments of this application or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of this application. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0056] Figure 1 shows the XRD pattern of the sodium iron phosphate pyrophosphate composite material prepared in Example 1.

[0057] Figure 2 is a SEM image of the sodium iron phosphate pyrophosphate composite material prepared in Example 1.

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

[0059] Figure 4 shows the XRD pattern of the sodium iron phosphate pyrophosphate composite material prepared in Example 2.

[0060] Figure 5 is a SEM image of the sodium iron phosphate pyrophosphate composite material prepared in Example 2.

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

[0062] Figure 7 shows the XRD pattern of the sodium iron phosphate pyrophosphate composite material prepared in Example 3.

[0063] Figure 8 is a SEM image of the sodium iron phosphate pyrophosphate composite material prepared in Example 3.

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

[0065] Figure 10 shows the XRD pattern of the sodium iron phosphate pyrophosphate composite material prepared in Example 4.

[0066] Figure 11 is a SEM image of the sodium iron phosphate pyrophosphate composite material prepared in Example 4.

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

[0068] Figure 13 shows the XRD pattern of the sodium iron phosphate pyrophosphate composite material prepared in Comparative Example 1.

[0069] Figure 14 is a SEM image of the sodium iron phosphate pyrophosphate composite material prepared in Comparative Example 1.

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

[0071] Figure 16 shows the XRD pattern of the sodium iron phosphate pyrophosphate composite material prepared in Comparative Example 2.

[0072] Figure 17 is a SEM image of the sodium iron phosphate pyrophosphate composite material prepared in Comparative Example 2.

[0073] [Correction 13.12.2024 according to Rule 91] Figure 18-1 shows the charge-discharge curves of the button cell assembled from the sodium iron phosphate pyrophosphate composite material prepared in Comparative Example 2. [0073.1] [Correction 13.12.2024 according to Rule 91] Figure 18-2 is a cycle performance diagram of the coin cell assembled from the sodium iron phosphate pyrophosphate composite material prepared in Comparative Example 2. Detailed Implementation

[0074] To facilitate understanding of this application, the following description will be provided in more full and detailed form with reference to the accompanying drawings and preferred embodiments. However, the scope of protection of this application is not limited to the specific embodiments described below.

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

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

[0077] Example 1

[0078] (1) Weigh out NH4FePO4·H2O, Na2CO3, Na4P2O7 and glucose at a molar ratio of 3:0.96:0.54.

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

[0080] (3) After drying the slurry after sand milling, the slurry was heated at a rate of 10℃ / min and sintered at 500℃ for 8 hours in an inert gas atmosphere to obtain the sodium iron phosphate pyrophosphate composite material Na4Fe. 2.94(PO4)2P2O7@C.

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

[0082] [Correction 13.12.2024 according to Rule 91] (4) Sodium iron pyrophosphate composite material Na4Fe prepared by the above method 2.94 (PO4)2P2O7@C was assembled with a sodium metal sheet to form a coin cell for testing. The coin cell assembly included Na4Fe as the positive electrode material. 2.94 The (PO4)2P2O7@C coating is applied to aluminum foil. The negative electrode material is metallic sodium. The separator used is Whatman GF / B. The electrolyte formulation is 1M NaClO4 dissolved in V% EC:PC = 1:1. The battery charge / discharge voltage range is 1.7–3.7V. The charge / discharge curves and cycle performance graphs are shown in Figures 3-1 and 3-2. Its electrochemical 0.1C discharge capacity is 102.3 mAh / g, and the capacity retention rate after 100 cycles at 1C is 96.8%.

[0083] Example 2

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

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

[0086] (3) After drying the slurry after sand milling, the mixture was heated at a rate of 13℃ / min and sintered at 530℃ for 10h in an inert gas atmosphere to obtain the sodium iron phosphate pyrophosphate composite material Na4Fe. 2.97(PO4)2P2O7@C. The XRD pattern after baseline subtraction 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 mainly distributed in the range of 100–200 nm, while the secondary particles are formed by the agglomeration of primary particles and are mainly distributed in the range of 300–800 nm. The carbon content, measured by a carbon-sulfur analyzer, is 1.8 wt%. The Ir of the material in this embodiment is 12.6.

[0087] [Correction 13.12.2024 according to Rule 91] (4) Sodium iron pyrophosphate composite material Na4Fe prepared by the above method 2.97 (PO4)2P2O7@C was assembled with a sodium metal sheet to form a coin cell for testing. The coin cell assembly included Na4Fe as the positive electrode material. 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 to 3.7V. The charge and discharge curves and cycle performance graphs 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) Weigh out NH4FePO4·H2O, Na2CO3, Na4P2O7 and glucose at a total mass of 10% of the above substances according to the stoichiometric ratio of 3:0.98:0.52.

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

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

[0092] (4) The sodium ferric phosphate pyrophosphate composite material Na4Fe prepared by the above method 2.97 (PO4)2P2O7@C was assembled with a sodium metal sheet to form a coin cell for testing. The coin cell assembly included Na4Fe as the positive electrode material. 2.97(PO4)2P2O7@C is coated on aluminum foil, the negative electrode material is metallic sodium, the separator 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 to 3.7V. The charge and discharge curves and cycle performance 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) Weigh out NH4FePO4·H2O, Na2CO3, Na4P2O7 and glucose, which together account for 20% of the total mass of the above substances, according to a stoichiometric ratio of 3:1:0.5.

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

[0096] (3) After drying the slurry after sand milling, it was sintered at 550℃ for 12h in an inert gas atmosphere at a rate of 15℃ / min to obtain the sodium iron phosphate pyrophosphate composite material Na4Fe3(PO4)2P2O7@C. The XRD pattern after baseline subtraction is shown in Figure 10, and the SEM image is shown in Figure 11. The primary particles of the sodium iron phosphate pyrophosphate composite material are mainly distributed in the range of 100–200 nm, while the secondary particles are formed by the agglomeration of primary particles and are mainly distributed in the range of 300–800 nm. The carbon content was measured to be 3 wt% using a carbon-sulfur analyzer, and the Ir of the material in this embodiment was 10.7.

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

[0098] Comparative Example 1

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

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

[0101] (3) After drying the slurry after sand milling, the temperature was increased in an inert gas atmosphere at a rate of 13℃ / min in the temperature ranges of 25-300℃ and 400-530℃, while the temperature was increased at a rate of 2℃ / min in the range of 300-400℃. The sintering was then carried out at 530℃ for 12 hours 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 of the material is 5.6.

[0102] Although the same raw material ratio as in Examples 2 and 3 was used in this comparative example to control the formation of impurity phases, the final product did not have a definite chemical formula. Therefore, the above-mentioned x-containing chemical formula was used.

[0103] [Correction 13.12.2024 according to Rule 91] (4) The material Na4Fe prepared by the above method 3(1-x) (PO4)2P2O7@C was assembled with a sodium metal sheet to form a coin cell for testing. The coin cell assembly included Na4Fe as the positive electrode material. 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 to 3.7V. The charge and discharge curves and cycle performance graphs 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) Weigh out NH4FePO4·H2O, Na2CO3, Na4P2O7 and glucose at 10% of the total mass of the above substances according to the stoichiometric ratio of 3:0.94:0.56.

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

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

[0108] [Correction 13.12.2024 according to Rule 91] (4) The material Na4Fe prepared by the above method 2.91 (PO4)2P2O7@C was assembled with a sodium metal sheet to form a coin cell for testing. The coin cell assembly included Na4Fe as the positive electrode material. 2.91 (PO4)2P2O7@C is coated on aluminum foil, the negative electrode material is metallic sodium, the separator 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 to 3.7V. The charge and discharge curves and cycle performance graphs are shown in Figures 18-1 and 18-2. The 0.1C discharge capacity is 98.4 mAh / g, and the capacity retention rate after 100 cycles at 1C is 93.7%.

[0109] Table 1

[0110] The electrochemical performance 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, i.e., when the content of the spinel-type NaFePO4 phase in the obtained sodium iron phosphate pyrophosphate composite material is within a certain range, the corresponding electrochemical performance, such as battery capacity and cycle performance, is better. Based on this finding, it can be used to evaluate and optimize the process parameters of the solid-state method for preparing composite sodium iron phosphate in this application. By optimizing the raw material ratio in step (1), the heating rate and sintering temperature and holding time in step (2), the method for preparing composite sodium iron phosphate can be optimized.

[0111] Regarding the 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 extended the NaFePO4 phase formation temperature zone time during heating, and the final Ir was too low, resulting in poor performance. This shows the feasibility and rationality of controlling the heating rate to reduce the NaFePO4 phase.

[0113] Regarding the optimization of process parameters in Comparative Example 2:

[0114] Examples 1 and 2 are equivalent to the optimized parameter results of Comparative Example 2. Comparative Example 2 shows that the NaFePO4 phase is also one of the structural phases of the 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 was further reduced (by 3%), and the Ir was too high and the performance was poor. The decline in cycle performance indirectly shows that the structural stability of the material itself was reduced under this condition.

[0115] The above description is only a preferred embodiment of this application. It should be noted that for those skilled in the art, several improvements and modifications can be made without departing from the principle of this application, and these improvements and modifications should also be considered within the scope of protection of this application.

Claims

1. A sodium iron phosphate pyrophosphate composite material, wherein, The sodium iron phosphate pyrophosphate composite material has a spinel-type NaFePO4 phase. XRD diffraction of the sodium iron phosphate pyrophosphate composite material under CuKα radiation revealed characteristic diffraction peaks T (for sodium iron phosphate pyrophosphate) and O (for spinel-type NaFePO4) at 2θ of 33.6±0.2° and 32.8±0.2°, respectively, with peak intensities I0 and I1, respectively. T and I O Let the diffraction peak intensity ratio Ir = I T / I O , where 6.19≤Ir≤13.

2. The sodium iron phosphate pyrophosphate composite material as described in claim 1, wherein, It includes sodium iron phosphate pyrophosphate and a carbon layer coating the surface of the sodium iron phosphate pyrophosphate, with the chemical formula Na₄Fe₂O₃. 3(1-x) (PO4)2P2O7@C, where 0≤x≤0.

02.

3. The sodium iron phosphate pyrophosphate composite material as described in claim 1 or 2, wherein, The carbon content of the sodium iron phosphate pyrophosphate composite material is 2-5 wt%.

4. A preparation process for a sodium iron phosphate pyrophosphate composite material, wherein, Includes the following steps: (1) Weigh out the synthetic Na4Fe according to the preset feeding ratio. 3(1-x) (PO4)2P2O7 raw material and carbon source are wet-milled and mixed to obtain a mixture. (2) After drying the mixture, it is sintered under a protective atmosphere according to preset sintering parameters to obtain the sodium iron phosphate pyrophosphate composite material Na4Fe. 3(1-x) (PO4)2P2O7@C; Where 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 for (PO4)2P2O7 include an iron source and a sodium source; the iron source is ferrous ammonium phosphate, and the sodium source includes sodium pyrophosphate and a sodium source that does not contain phosphorus.

6. The preparation process according to claim 5, wherein, The sodium source that does not contain phosphorus 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 ferrous ammonium phosphate, a phosphorus-free sodium source, and 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 carbon source used is based on the synthesis of Na₄Fe. 3(1-x) The raw material of (PO4)2P2O7 accounts for 5-20% of the mass.

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 met: The heating rate for sintering is 10–15 °C / min; The sintering temperature is 500–550°C; The sintering time is 8 to 12 hours.

10. An evaluation method for sodium iron phosphate pyrophosphate composite materials, wherein, Includes the following steps: S1. The sodium iron phosphate pyrophosphate composite material was subjected to XRD diffraction test under CuKα radiation, and the XRD diffraction pattern was obtained. The corresponding performance test value was calculated based on the peak intensity of the required characteristic diffraction peaks. S2. Obtain performance evaluation results by comparing performance test values ​​and performance reference values.

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

12. An optimized method for preparing sodium iron phosphate pyrophosphate composite materials, wherein, The evaluation of the sodium iron phosphate pyrophosphate composite material prepared by any one of the preparation processes of claims 4 to 9 or the sodium iron phosphate pyrophosphate composite material according to any one of claims 1 to 3, using the evaluation method according to claim 10 or 11, further includes the following steps: S3. By comparing the performance test value and the performance reference value, determine 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) and (2). 2) Steps 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.

13. The optimization method as described in claim 12, wherein, Based on the comparison results, optimize and adjust the process parameters in step (1) or (2), including the amount of iron source added in the preset feeding 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 bound of the Ir interval, reduce the amount of iron source added and / or increase the heating rate; when Ir' is greater than the upper bound of the Ir interval, increase the amount of iron source added and / or decrease the heating rate.

15. The application of the sodium iron phosphate pyrophosphate composite material according to any one of claims 1 to 3, or the sodium iron phosphate pyrophosphate composite material prepared by any one of claims 4 to 9, or the sodium iron phosphate pyrophosphate composite material evaluated as superior by the evaluation method according to claim 10 or 11, or the sodium iron phosphate pyrophosphate composite material prepared by the optimized process according to the optimization method according to any one of claims 12 to 14, wherein, Application of sodium-ion battery cathode material in sodium-ion batteries.

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