Composite electrode materials based on sodium ferric phosphate (sodium ferric phosphate), their preparation methods and applications

Abstract

This invention relates to a composite electrode material based on sodium iron phosphate of the borax-iron type, its preparation method, and its application. The composite electrode material has a composite structure of carbon-coated NaFePO4 / Na2FePO4F composite crystal phases. The NaFePO4 / Na2FePO4F composite crystal phase is formed by in-situ composite of NaFePO4 and Na2FePO4F phases, with a heterogeneous interface between the two phases. The mass ratio of NaFePO4 to Na2FePO4F is 65-85:15-35. The beneficial effects are that the composite electrode material based on sodium iron phosphate of the borax-iron type prepared by this invention, when used as the positive electrode of sodium-ion batteries, exhibits high capacity and good cycle performance, which can promote the application of sodium iron phosphate of the borax-iron type electrode material and has great potential application value.

Description

Technical Field

[0001] This invention relates to the field of electrochemical materials technology, and in particular to a composite electrode material based on sodium iron phosphate of the phosphate rock type, its preparation method and application. Background Technology

[0002] With the increasing demands for global energy structure transformation and sustainable development, lithium-ion batteries (LIBs) dominate the fields of electric vehicles and energy storage. However, the scarcity of lithium resources and safety concerns make it difficult to meet the needs of large-scale energy storage applications. Sodium-ion batteries (SIBs), which work on a similar principle to lithium-ion batteries, are gradually becoming a research hotspot due to the abundance of sodium resources and low cost.

[0003] Among them, maricite-NaFePO4 (m-NFP), a type of sodium iron phosphate, has attracted widespread research interest due to its simple synthesis process and ease of large-scale production. However, this material lacks continuous sodium ion diffusion channels, resulting in extremely low electrochemical activity, which makes it difficult to meet the needs of practical applications.

[0004] In recent years, by employing microstructural manipulation techniques such as nanostructuring, amorphization, and lattice doping, the intrinsic inertia of m-NFPs has been gradually overcome, successfully activating their electrochemical activity. To address the low intrinsic electronic conductivity of m-NFPs, they have been further modified by composite modification with carbon-based materials (such as porous carbon, nitrogen-doped carbon fibers, and reduced graphene oxide). However, while existing modification strategies such as nanostructuring, amorphization, lattice doping, and carbon composites have improved the electrochemical performance of m-NFPs, they have not yet solved the core problem of their dual limitation on ion / electron transport, nor have they fundamentally resolved the inherent contradiction between energy density and kinetics. Therefore, it is difficult to truly advance the practical application of m-NFP materials. Summary of the Invention

[0005] (a) Technical problems to be solved In view of the above-mentioned shortcomings and deficiencies of the prior art, the present invention provides a composite electrode material based on sodium iron phosphate of phosphate rock type, its preparation method and application, which solves the technical problem that the ion / electron transport of electrode materials prepared by existing sodium iron phosphate of phosphate rock type is doubly limited, thus affecting the expanded application of sodium iron phosphate of phosphate rock type electrode materials.

[0006] (II) Technical Solution To achieve the above objectives, the main technical solutions adopted by the present invention include: In a first aspect, embodiments of the present invention provide a composite electrode material based on sodium iron phosphate of the phosphate rock type. The composite electrode material has a composite structure of carbon-coated NaFePO4 / Na2FePO4F composite crystal phase; Among them, the NaFePO4 / Na2FePO4F composite crystal phase is formed by the in-situ composite of NaFePO4 phase and Na2FePO4F phase, with a heterogeneous interface between the two phases; The mass ratio of NaFePO4 to Na2FePO4F is 65-85:15-35.

[0007] In a preferred embodiment of the present invention, the composite electrode material based on sodium iron phosphate of phosphate rock has a mass ratio of NaFePO4 to Na2FePO4F of 70-80:20-30.

[0008] In a preferred embodiment of the present invention, the composite electrode material based on sodium iron phosphate of the phosphate rock type has a mass ratio of NaFePO4 to Na2FePO4F of 73-77:23-27.

[0009] Secondly, embodiments of the present invention provide a method for preparing a composite electrode material based on sodium iron phosphate of the phosphate rock type, comprising the following steps: S1. Sodium source, iron source, phosphorus source, fluorine source, carbon source and solvent are ball-milled and mixed evenly, and then dried to obtain a premix. S2. In an inert gas atmosphere, the premix is ​​calcined for the first time and then for the second time to obtain a composite electrode material based on sodium iron phosphate of the phosphate rock type; the second calcination temperature is more than 300°C higher than the first calcination temperature.

[0010] Before the second calcination, the material after the first calcination can be compacted before the second calcination. The specific compaction process can be found in existing technology.

[0011] In a preferred embodiment of the present invention, in the preparation method S1, the sodium source is selected from at least one of sodium carbonate, sodium bicarbonate, and sodium fluoride; the iron source is selected from at least one of ferrous oxalate and ferric phosphate; the phosphorus source is selected from at least one of ammonium dihydrogen phosphate and ferric phosphate; the fluorine source is selected from at least one of sodium fluoride, sodium tetrafluoroborate, and polytetrafluoroethylene; the carbon source is selected from at least one of citric acid, urea, glucose, and sucrose; and the solvent is selected from at least one of anhydrous ethanol and isopropanol.

[0012] In a preferred embodiment of the present invention, in the preparation method S1, the sodium source is sodium carbonate; the iron source is ferrous oxalate; the phosphorus source is ammonium dihydrogen phosphate; the fluorine source is sodium tetrafluoroborate; the carbon source is citric acid and urea; and the solvent is anhydrous ethanol.

[0013] In a preferred embodiment of the present invention, in the preparation method S1, the molar ratio of sodium carbonate to ferrous oxalate is 0.5-0.6:1; and the molar ratio of sodium tetrafluoroborate to ammonium dihydrogen phosphate is 5-35:100.

[0014] In a preferred embodiment of the present invention, in the preparation method S1, ball milling is performed for 5-8 hours at a speed of 350-400 rpm.

[0015] In a preferred embodiment of the present invention, in the preparation method S2, the temperature of the first calcination is 300-350℃ and the time is 3-6 h; the temperature of the second calcination is 600-650℃ and the time is 10-14 h.

[0016] Thirdly, embodiments of the present invention provide a sodium-ion battery comprising the composite electrode material based on sodium iron phosphate of the phosphate rock type, or the composite electrode material based on sodium iron phosphate of the phosphate rock type prepared by the preparation method.

[0017] (III) Beneficial Effects The beneficial effects of this invention are as follows: The composite electrode material based on sodium iron phosphate of the phosphate-iron-sodium type, its preparation method, and its application are as follows: The composite electrode material based on sodium iron phosphate of the phosphate-iron-sodium type has a composite structure of carbon-coated NaFePO4 / Na2FePO4F composite crystal phases, with a heterogeneous interface formed between the NaFePO4 phase and the Na2FePO4F phase. The mass ratio of NaFePO4 to Na2FePO4F is 65-85:15-35. The heterogeneous structure formed by the NaFePO4 and Na2FePO4F phases effectively establishes a continuous m-NFP ion channel, significantly improving the specific capacity and cycle stability of the m-NFP electrode material. The electrode material prepared by the method of the composite electrode material has good crystallinity, is free of impurities, has low cost, and the process is simple and easy to scale up. Compared with the prior art, the composite electrode material based on sodium iron phosphate of the phosphate-iron-sodium type prepared by this invention, when used as the positive electrode of sodium-ion batteries, has high capacity and good cycle performance, which can promote the application of sodium iron phosphate of the phosphate-iron-sodium type electrode material and has great potential application value. Attached Figure Description

[0018] Figure 1 The XRD patterns of the electrode materials prepared in the embodiments and comparative examples of this invention are shown below. Figure 2 The XRD refinement results are shown for the sample prepared in Comparative Example 3 of this invention. Figure 3 The XRD refinement results are shown for the sample prepared in Example 1 of this invention. Figure 4 The XRD refinement results are shown for the sample prepared in Comparative Example 4 of this invention. Figure 5 This is a high-resolution TEM image of the electrode material prepared in Example 1 of the present invention, wherein b is an enlarged view of the rectangular region in a; Figure 6 The charge-discharge curve of the electrode material prepared in Comparative Example 1 of this invention; Figure 7 The charge-discharge curve of the electrode material prepared in Comparative Example 2 of this invention; Figure 8 The charge-discharge curve of the electrode material prepared in Comparative Example 3 of this invention; Figure 9 The charge-discharge curve of the electrode material prepared in Example 1 of this invention; Figure 10 The charge-discharge curve of the electrode material prepared in Comparative Example 4 of this invention; Figure 11 Cycle performance curves of sodium-ion batteries prepared using electrode materials prepared for embodiments and comparative examples of the present invention. Detailed Implementation

[0019] To better explain and facilitate understanding of the present invention, the present invention will be described in detail below with reference to the accompanying drawings and specific embodiments.

[0020] To better understand the above technical solutions, exemplary embodiments of the present invention will be described in more detail below with reference to the accompanying drawings. Although exemplary embodiments of the present invention are shown in the drawings, it should be understood that the present invention can be implemented in various forms and should not be limited to the embodiments set forth herein. Rather, these embodiments are provided so that the present invention can be understood more clearly and thoroughly, and that the scope of the present invention can be fully conveyed to those skilled in the art.

[0021] Example 1

[0022] This embodiment provides a method for preparing a composite electrode material based on sodium iron phosphate of the phosphate rock type. The specific steps are as follows: (1) Place 0.53g sodium carbonate, 1.80g ferrous oxalate dihydrate, 1.15g ammonium dihydrogen phosphate, 0.35g citric acid, 0.18g urea, 0.16g NaBF4 and 50ml anhydrous ethanol in a ball mill and ball mill for 6 h at a speed of 400 rpm. After drying, a premix is ​​obtained. (2) The premix was calcined at 300℃ for 4 h (N2 atmosphere) and then calcined at 600℃ for 12 h (N2 atmosphere) to obtain the composite electrode material NFP / F@C-2 of sodium iron phosphate of sodium phosphate type.

[0023] The XRD pattern of the NFP / F@C-2 prepared in this embodiment can be found in [reference needed]. Figure 1As can be seen, most of the main peaks are consistent with the NaFePO4 (NFP(PDF#01-071-5040)) card, but diffraction peaks corresponding to Na2FePO4F (NFPF(ICSD#167044)) appear at positions of 12.8° and 30.3°. Figure 3 The mass ratio of NFP to NFPF phases was determined to be 75.9:24.1 by XRD refinement.

[0024] Figure 5 This is a high-resolution TEM image of the NFP / F@C-2 prepared in this embodiment, which clearly shows the lattice fringes corresponding to the NFP (NaFePO4) phase and the NFPF (Na2FePO4F) phase, as well as the heterostructure between them.

[0025] Application Example 1 This application example evaluates the electrochemical performance of the NFP / F@C-2 composite electrode material of sodium iron phosphate of the sodium ferrophosphate type prepared in Example 1: The positive electrode of the assembled coin cell is prepared by coating a uniform slurry containing active material (NFP / F@C-2), acetylene black and PVDF binder (mass ratio 7:2:1) onto an aluminum foil current collector.

[0026] The electrodes were vacuum dried at 110°C for 6 hours and then punched into discs with a diameter of 14 mm. The battery was assembled in an argon-filled glove box, using a sodium metal anode, a glass microfiber separator (Whatman), and a 1M NaClO4 / EC:DMC:EMC (volume ratio 1:1:1) electrolyte containing 2 vol.% fluoroethylene carbonate additive.

[0027] The prepared sodium-ion battery was subjected to charge-discharge tests, and the test results are as follows: Figure 9 At 0.1C (1 C = 155 mAh g), -1 Charge-discharge tests were conducted at a current density of [value missing], and the highest specific capacity was observed during the first discharge cycle, reaching 116.5 mAh g⁻¹. -1 See the corresponding cycle performance curves for sodium-ion batteries. Figure 11 .

[0028] Comparative Example 1 This comparative example describes a method for preparing an electrode material. The difference between this comparative example and Example 1 is that: (1) Place 0.53g sodium carbonate, 1.80g ferrous oxalate dihydrate, 1.15g ammonium dihydrogen phosphate, 0.35g citric acid, 0.18g urea, 0.09g boric acid and 50ml anhydrous ethanol in a ball mill and ball mill for 6 h at a speed of 400 rpm. After drying, a premix is ​​obtained. The remaining steps are the same; The prepared electrode material is denoted as NFP / C.

[0029] Figure 1 The XRD pattern of the prepared NFP / C is shown. It can be seen that the XRD pattern is consistent with the NFP (PDF#01-071-5040) card.

[0030] Comparative Example 2 This comparative example describes a method for preparing an electrode material. The difference between this comparative example and Example 1 is that: (1) Place 0.53g sodium carbonate, 1.80g ferrous oxalate dihydrate, 1.15g ammonium dihydrogen phosphate, 0.42g NaF, 0.35g citric acid, 0.18g urea, 0.09g boric acid and 50ml anhydrous ethanol in a ball mill and ball mill for 6 h at a speed of 400 rpm. After drying, a premix is ​​obtained. The remaining steps are the same; The prepared electrode material is denoted as NFPF / C.

[0031] Figure 1 The XRD pattern of the prepared NFPF / C is shown. It can be seen that the XRD pattern is consistent with the NFPF (ICSD#167044) card.

[0032] Comparative Example 3 This comparative example describes a method for preparing an electrode material. The difference between this comparative example and Example 1 is that: (1) Place 0.53g sodium carbonate, 1.80g ferrous oxalate dihydrate, 1.15g ammonium dihydrogen phosphate, 0.35g citric acid, 0.18g urea, 0.09g NaBF4 and 50ml anhydrous ethanol in a ball mill and ball mill for 6 h at a speed of 400 rpm. After drying, a premix is ​​obtained. The remaining steps are the same; The prepared electrode material is denoted as NFP / F@C-1.

[0033] Figure 1 The XRD pattern of the prepared NFP / F@C-1 is shown. It can be seen that most of the main peaks of the XRD pattern are consistent with the NFP (PDF#01-071-5040) card, but a diffraction peak corresponding to NFPF (ICSD#167044) appears at the position of 30.3°. Figure 2 XRD refinement revealed that the mass ratio of NFP to NFPF was 93.1:6.9.

[0034] Comparative Example 4 This comparative example describes a method for preparing an electrode material. The difference between this comparative example and Example 1 is that: (1) Place 0.53g sodium carbonate, 1.80g ferrous oxalate dihydrate, 1.15g ammonium dihydrogen phosphate, 0.35g citric acid, 0.18g urea, 0.25g NaBF4 and 50ml anhydrous ethanol in a ball mill and ball mill for 6 h at a speed of 400 rpm. After drying, a premix is ​​obtained. The remaining steps are the same; The prepared electrode material is denoted as NFP / F@C-3.

[0035] Figure 1 The XRD pattern of the prepared NFP / F@C-3 is shown. It can be seen that most of the main peaks of the XRD pattern are consistent with the NFP (PDF#01-071-5040) card, but diffraction peaks corresponding to NFPF (ICSD#167044) appear at positions of 12.8° and 30.3°, and these diffraction peaks are stronger. Figure 4 The mass ratio of NFP to NFPF was determined to be 44.2:55.8 by XRD refinement.

[0036] Comparative Example 5 Electrochemical performance of the electrode materials prepared in Comparative Examples 1 to 4 was evaluated: coin cells were assembled, and the positive electrode was prepared by coating an aluminum foil current collector with a uniform slurry containing four active materials (NFP@C, NFPF@C, NFP / F@C-1 and NFP / F@C-3), acetylene black and PVDF binder (mass ratio 7:2:1).

[0037] The electrodes were vacuum dried at 110°C for 6 hours and then punched into discs with a diameter of 14 mm. The battery was assembled in an argon-filled glove box, using a sodium metal anode, a glass microfiber separator (Whatman), and a 1M NaClO4 / EC:DMC:EMC (volume ratio 1:1:1) electrolyte containing 2 vol.% fluoroethylene carbonate additive.

[0038] Charge-discharge tests were conducted on four types of prepared sodium-ion batteries. The test results for the sodium-ion battery containing NFP@C are as follows: Figure 6 At 0.1C (1 C = 155 mAh g), -1 Charge-discharge tests were conducted at a current density of [value missing], and the first discharge specific capacity was only 51.2 mAh g. -1 The test results for sodium-ion batteries containing NFPF@C are as follows: Figure 7 At 0.1C (1 C = 155 mAh g), -1 Charge-discharge tests were conducted at a current density of [value missing], and the first discharge specific capacity was 97.2 mAh g. -1 The test results for sodium-ion batteries containing NFP / F@C-1 are as follows: Figure 8At 0.1C (1 C = 155 mAh g), -1 Charge-discharge tests were conducted at a current density of [value missing], and the first discharge specific capacity was 59.5 mAh g. -1 The test results for sodium-ion batteries containing NFP / F@C-3 are as follows: Figure 10 At 0.1C (1 C = 155 mAh g), -1 Charge-discharge tests were conducted at a current density of [value missing], and the first discharge specific capacity was 99.3 mAh g. -1 See the cycle performance curves for the four types of sodium-ion batteries. Figure 11 .

[0039] The analysis based on the above embodiments, application examples, and comparative examples is as follows: In Example 1, the molar ratio of NFP to NFPF was 75.9:24.1, exhibiting optimal electrochemical performance (116.5 mAh g⁻¹). -1 The conductivity was significantly higher than that of the pure-phase materials prepared in Comparative Examples 1 and 2. This indicates that the heterogeneous interface formed between NFP and NFPF effectively improves the electronic conductivity and sodium ion transport kinetics of the material.

[0040] A comparison of the electrode materials prepared in Example 1 with those prepared in Comparative Examples 3 and 4 shows that excessive or insufficient NaBF4 addition affects the ratio of NFP and NFPF phases in the electrode material, thereby affecting the discharge specific capacity and battery cycle performance. This is because excessive NFPF phase disrupts the optimal heterostructure balance.

[0041] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

[0042] The "range" disclosed in this application is defined by a lower limit and an upper limit. A given range is defined by selecting a lower limit and an upper limit, which define the boundaries of a particular range. Ranges defined in this way can include or exclude endpoints and can be arbitrarily combined; that is, any lower limit can be combined with any upper limit to form a range. For example, if ranges of 60-120 and 80-110 are listed for a specific parameter, it is expected that ranges of 60-110 and 80-120 are also included. Furthermore, if minimum range values ​​of 1 and 2 are listed, and if maximum range values ​​of 3, 4, and 5 are listed, then the following ranges are all expected: 1-3, 1-4, 1-5, 2-3, 2-4, and 2-5. In this application, unless otherwise stated, the numerical range "ab" represents a shortened representation of any combination of real numbers between a and b, where a and b are real numbers. For example, the numerical range "0-5" indicates that all real numbers between "0-5" have been listed in this article; "0-5" is simply a shortened representation of these numerical combinations. Furthermore, when a parameter is stated as an integer ≥2, it is equivalent to disclosing that the parameter is, for example, an integer such as 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, etc.

[0043] Unless otherwise specified, all embodiments and optional embodiments of this application can be combined to form new technical solutions.

[0044] Unless otherwise specified, all technical features and optional technical features of this application may be combined to form new technical solutions.

[0045] Unless otherwise specified, all steps in this application may be performed sequentially or randomly, for example, sequentially. For instance, the method includes steps (a) and (b), indicating that the method may include steps (a) and (b) performed sequentially, or it may include steps (b) and (a) performed sequentially. For instance, the method may also include step (c), indicating that step (c) may be added to the method in any order; for example, the method may include steps (a), (b), and (c), or it may include steps (a), (c), and (b), or it may include steps (c), (a), and (b), etc.

[0046] Unless otherwise specified, the terms "comprising" and "including" as used in this application can be open-ended or closed-ended. For example, "comprising" and "including" can mean that other components not listed may also be included, or that only the listed components may be included.

[0047] Unless otherwise specified, the term "or" is inclusive in this application. For example, the phrase "A or B" means "A, B, or both A and B". More specifically, the condition "A or B" is satisfied by any of the following conditions: A is true (or exists) and B is false (or does not exist); A is false (or does not exist) and B is true (or exists); or both A and B are true (or exist).

Claims

1. A composite electrode material based on sodium iron phosphate of the phosphate rock type, characterized in that, The composite electrode material has a composite structure of carbon-coated NaFePO4 / Na2FePO4F composite crystal phase; Among them, the NaFePO4 / Na2FePO4F composite crystal phase is formed by the in-situ composite of NaFePO4 phase and Na2FePO4F phase, with a heterogeneous interface between the two phases; The mass ratio of NaFePO4 to Na2FePO4F is 65-85:15-35.

2. The composite electrode material based on sodium iron phosphate of ferrophosphate type as described in claim 1, characterized in that, The mass ratio of NaFePO4 to Na2FePO4F is 70-80:20-30.

3. The composite electrode material based on sodium iron phosphate of ferrophosphate type as described in claim 1, characterized in that, The mass ratio of NaFePO4 to Na2FePO4F is 73-77:23-27.

4. A method for preparing a composite electrode material based on sodium iron phosphate of the phosphate rock type as described in any one of claims 1 to 3, characterized in that, Includes the following steps: S1. Sodium source, iron source, phosphorus source, fluorine source, carbon source and solvent are ball-milled and mixed evenly, and then dried to obtain a premix. S2. In an inert gas atmosphere, the premix is ​​calcined for the first time and then for the second time to obtain a composite electrode material based on sodium iron phosphate of the phosphate rock type; the second calcination temperature is more than 300°C higher than the first calcination temperature.

5. The preparation method according to claim 4, characterized in that, In S1, the sodium source is selected from at least one of sodium carbonate, sodium bicarbonate, and sodium fluoride; the iron source is selected from at least one of ferrous oxalate and ferric phosphate; the phosphorus source is selected from at least one of ammonium dihydrogen phosphate and ferric phosphate; the fluorine source is selected from at least one of sodium fluoride, sodium tetrafluoroborate, and polytetrafluoroethylene; the carbon source is selected from at least one of citric acid, urea, glucose, and sucrose; and the solvent is selected from at least one of anhydrous ethanol and isopropanol.

6. The preparation method according to claim 5, characterized in that, In S1, the sodium source is sodium carbonate; the iron source is ferrous oxalate; the phosphorus source is ammonium dihydrogen phosphate; the fluorine source is sodium tetrafluoroborate; the carbon source is citric acid and urea; and the solvent is anhydrous ethanol.

7. The preparation method according to claim 6, characterized in that, In S1, the molar ratio of sodium carbonate to ferrous oxalate is 0.5-0.6:1; the molar ratio of sodium tetrafluoroborate to ammonium dihydrogen phosphate is 5-35:

100.

8. The preparation method according to claim 4, characterized in that, In S1, during ball milling, the ball milling time is 5-8 hours, and the rotation speed is 350-400 rpm.

9. The preparation method according to claim 4, characterized in that, In S2, the first calcination temperature is 300-350℃ and the time is 3-6 h; the second calcination temperature is 600-650℃ and the time is 10-14 h.

10. A sodium-ion battery, characterized in that, It comprises the composite electrode material based on sodium iron phosphate of phosphate rock type as described in any one of claims 1 to 3, or the composite electrode material based on sodium iron phosphate of phosphate rock type prepared by the preparation method described in any one of claims 4 to 9.

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